Before each use, take a moment to inspect the insulin prior to drawing it into the syringe; clear insulins should appear not discolored and clear; suspended insulins should be uniform in their cloudiness. 
Do not use the insulin if:
See Insulin problems for more information about "bad" insulin.
Damaged Insulin: Insulin that is getting too old, or has been dropped or shaken or mishandled, or exposed to a lot of light or heat, will be less effective than before.
Freezing  destroys the molecules of ANY insulin; any that has either been frozen or is suspected of having been frozen should not be used. Insulin which has been frozen will not be able to do an effective job of controlling blood glucose. 
Check for discoloration or floating objects in the insulin -- it may also be contaminated.
It's also possible that the new or newer vial from the pharmacy may be flawed. If you've recently started it and are having problems, this might be the case. Taking down the lot number and getting a new vial that has a different batch/lot number should take care of this.
Frosted insulin: If insulin is subjected to temperature extremes, such as freezing or overheating, the insulin can precipitate  on the vial's walls, giving it a frosty or frosted appearance. Another term used to describe this is flocculation.  In the photo above, the insulin vial on the right is a visual example of what a frosted vial would look like. You can see the precipitated insulin clinging to the sides of it.
If you made a mistake and forgot to put the insulin back into the refrigerator, even for several hours, there should be no problem. Many keep their insulin (see instructions for your brand) at room temperature all the time.  When comparing it to insulin which is in the fridge except when used, it may be more likely to have slight potency loss. When a vial has been left out of the refrigerator for some time, it needs to be monitored for any signs of frosting;  in the photo, the vial on the left is an example of frosting. If the insulin was exposed to heat or direct light for a while when it was out of the refrigerator, or shaken vigorously or dropped a long way, the best thing to do would be to start with a new vial. 
Insulin can become contaminated by reusing syringes. Syringe needles have a silicon coating designed to make injections less painful. Re-use not only blunts the needle, it also wears away the protective silicon coating; white precipitates  can form in the vial from the silicon, possibly interfering with the action of the insulin.  
According to Diabetes Health, the best place to keep insulin in your fridge is in the door of it-"Why? Because items stored in the main portion of a refrigerator may get pushed to the back, where there is a possibility of frost or even freezing. Storing the vials in the door will ensure that does not happen." 
Freezing is more destructive to insulin than higher temperatures. 
Injecting cold from the refrigerator insulin  can sting, regardless of what species, type or brand.  Bringing the insulin to room temperature by removing it from the fridge before actually using it can help avoid painful injections. Warming the capped insulin syringe with your hands can have the same effect. Some people tuck the capped and filled syringe under their arm for a few minutes to warm it before use.
Do NOT attempt to warm insulin using a stove, microwave, etc.; you may destroy the insulin by doing so .
If necessary, syringes can be prefilled and stored for up to two weeks in the refrigerator. When storing prefilled syringes, they need to be kept in a position that keeps the capped needle pointing upward so there's no chance of any insulin plugging the needle at actual injection time.  If the insulin in the syringes is one of the cloudy ones, you will need to re-suspend the insulin in them before use by rolling the syringe gently between your hands just as is done before drawing insulin for immediate use.
There are times when you may want to prefill a syringe with a known dose and leave it for yourself or someone else to use later. If doing so, keep in mind:
- Recap the needle with great care -- needles can easily go through the side of the cap into your fingers.
- Store the recapped needle point-up to avoid clogging the needle.
- If the insulin needs refrigeration normally, so do the prefilled syringes.
- The "cloudy" insulins (NPH, Lente/Ultralente, PZI, Mixes) need to be re-suspended before use. Gently roll the syringe back and forth in your hands as with the vial. Improperly suspended insulin may lose some or all of its effectiveness.
- Some insulins, including Lantus and Vetsulin, will react with the syringe's inner coating over time and lose effectiveness, or even lose their time-delay action. Check with your insulin manufacturer if prefilling syringes is permitted.
- According to Novo Nordisk,  prefilled syringes containing any of their Novolin, Actrapid, Insulatard, Actraphane, and Mixtard insulins can be kept refrigerated for 30 days. Eli Lilly says any of their Humulin insulins in prefilled syringes are good for 21 days when refrigerated.
Do NOT wipe the needle with alcohol as it removes the protective coating which makes injection easier and less painful. 
But varying the injection site can cause variability in action profile, too. This page illustrates  illustrates the most common areas humans with diabetes inject insulin and explains how absorption differs in various areas of the human body. This is true for ALL insulins. 
The new shot area needn't be very far from where the last shot was given--the distance of the width of 2 fingers will do fine as a measure. 
Most of us dealing with pet diabetes vary the side we give the injections in--right side mornings and left side evenings, for example. This is another help in avoiding giving shots in the same areas. 
Many people give insulin shots in the scruff of the pet's neck, which is now considered to be a less than optimum choice. The neck area provides poor insulin absorption, due to it not having many capillaries, veins. etc. (vascularization).
Other sites suggested by Dr. Greco include the flank and armpit. 
Intervet recommends giving injections from just back of the shoulder blades to just in front of the hipbone on either side, from 1 to 2 inches from the middle of the back. 
Rubbing the site where you gave the shot increases insulin absorption. This means that the insulin is used faster than normal (no rubbing of the site) and might lead to a hypo. Rubbing the site increases blood flow to the area and this in turn speeds up absorption. 
- To eliminate bubbles: If drawing insulin from a vial, set the syringe plunger to the dosage you want to draw, put the needle into the top of the vial while the vial is still upright, and push all the air out of the syringe. This will maintain air pressure equilibrium in the vial once you draw the dose and, because the vial is upright, will not cause air bubbles to mix with the solution. Turn the vial upside down and draw the insulin slowly. See note.
- Some people prefer to gently jiggle their vials to make any air bubbles rise to the top, away from where the needle will draw.
From WebMD: 
"If you notice an air bubble inside the syringe, draw a little extra insulin into the tube. Then, remove the needle from the bottle and hold the syringe-needle apparatus with the needle pointing toward the ceiling. Tap or flick the insulin syringe until the air bubble rises, and then push the plunger to force the air out of the syringe and get rid of any extra insulin."
Some tips from Ed Bryant at National Federation for the Blind, who draws his own insulin.
"How to Get Air Bubbles Out of an Insulin Syringe: 
"There are techniques by which a blind diabetic may draw and mix insulin without drawing air into the syringe. Like many others, I have used them successfully for years. I first draw four or five units of regular insulin into the syringe and then inject all of it back into the vial. I then repeat the operation two more times. The fourth time, I draw the full amount of insulin needed from the first vial. Then, when I draw insulin from the second vial, I draw the exact amount needed. I have put this to the test; 100 repetitions without air bubbles. Diabetes Action Network former First Vice President Janet Lee twice performed the same test. In both cases the complete absence of air in the syringe was independently verified."
- If you should bend a needle while drawing insulin, discard the syringe and start again; don't try to straighten it out.  Inserting the needle all the way into the vial makes it less easy to bend. 
There are sometimes leakage problems, when some insulin is lost when the needle is removed from the skin.  Some possible reasons and "fixes" for this are holding the "pinch" or "squeeze" too long which you made to give the shot.
The skin, now with insulin under it, is still being "squeezed" as it was before the insulin went under the skin. The "pinch" forces some of the insulin back out from the newly-created hole in the skin.
Releasing the "squeeze" or "pinch" first, then counting to 10 before removing the needle from the skin may give the insulin time to penetrate the fat layer and prevent leakage. Short needles can also cause insulin leakage--switching to longer ones can also help. 
- ↑ Insulin Therapy-Stability & Storage. RxEd.org.
- ↑ 2.0 2.1 2.2 2.3 Keeping an Eye on Your Insulin. Diabetes Health (2001).
- ↑ Benson EA, Benson JW Jr, Fredlund PN, Mecklenburg RS, Metz R. (1988). Flocculation & Loss of Potency of Human NPH Insulin. Diabetes Care.
- ↑ 4.0 4.1 Playán J, Acha J, Navarro H, Sanz A, Guallar AM, Albero R. (1994). Flocculation of NPH Insulin. Revista Clinica Espanola-(English Translation).
- ↑ 5.0 5.1 5.2 Rosskamp, Ralf H. Park, Glen (1998). Long Acting Analog Insulins-Frosting Caused in NPH/Isophane Insulin By Heat/Cold. Journal-Diabetes.org.
- ↑ 6.0 6.1 Patchuki, Linda, et. al. (1 November 1987). Flocculation of Insulin Preparations. Annals of Internal Medicine.
- ↑ 7.0 7.1 Definition of Flocculation. Dorlands Medical Dictionary.
- ↑ 8.0 8.1 Diabetes Forecast-Storage & Safety-Frosting of NPH, Lente, Ultralente Insulins-Page 5. American Diabetes Association (2006).
- ↑ Humalog & Heat. Diabetesnet.com.
- ↑ 10.0 10.1 10.2 Injecting Insulin-Transcript of American Diabetes Association Videotape. American Diabetes Association (2003).
- ↑ Insulin FAQs. Pet-Diabetes.co.uk.
- ↑ Freezing Insulin; Injection Changes. Diabetes Forecast (August 2008).
- ↑ 13.0 13.1 Precipitation (chemistry). Wikipedia.
- ↑ Glossary. Sugardiabetes.net.
- ↑ 15.0 15.1 Insulin Update-Storage of Insulin. Nora Apothecary.
- ↑ Travelling With Insulin. FAQs.org.
- ↑ Tips on Caring for Diabetic Pets. Diabetic-help.com.
- ↑ Vetsulin-Technical Bulletin-Page 3. Intervet.
- ↑ Dowling, P. M. (1995). Insulin therapy in dogs and cats-. Canadian Veterinary Journal.
- ↑ Insulin Administration-Injecting Cold Insulin. Diabetes Journal (2002).
- ↑ Ask the D Team. Children With Diabetes (2005).
- ↑ Individualizing Insulin Therapy in Type 1 Diabetes Mellitus. Pharmacy Times (2005).
- ↑ Prefilled Syringes. BD Diabetes.
- ↑ Tips for Comfortable Injections. PetDiabetes.org.
- ↑ Hussein, Saleh Farag, Coates, Phillip, Green, Gill, Siddique, Haron (2006). Lipoatrophy can Happen With Any Subcutaneous Insulin. Endocrine Abstracts.
- ↑ 26.0 26.1 26.2 26.3 Heinemann, Lutz (January 2008). Variability of Insulin Action:Does It Matter?-page 40 (4 of 9). Insulin Journal.
- ↑ Common Human Insulin Injection Areas & Their Absorption Rates. BD Diabetes.
- ↑ Tips for Injecting Insulin. Joslyn Diabetes Center.
- ↑ Greco, Deborah. FAQ's About Diabetic Dogs. BD Diabetes.
- ↑ Better Medicine E-Newsletter. Intervet (June 2006).
- ↑ Vetsulin-Preparing Insulin & Giving Injection-Page 2. Intervet.
- ↑ Hirsch, Irl (2001). Insulin in the Hospital Setting-page 8. U of Washington Medical Center.
- ↑ How to Get Air Bubbles Out of Vetsulin (or any insulin). eHow.
- ↑ American Diabetes Association-Position Statement-Insulin Administration. American Diabetes Association (2002).
- ↑ Giving A Dog Insulin #7. WebMD.
- ↑ Bryant, Ed. Blind-Blind Diabetes Can Draw Insulin Safely. National Federation for the Blind.
- ↑ Diabetes Mellitus. Washington State University.
- ↑ Ask the D Team-Insulin Leaking From Injection Site. Children With Diabetes (2006).
- ↑ Griffin, Stacy, Hieronymus, Laura. Insulin Delivery Devices. Diabetes Self Management.