Diabetes and other body processes 

Cats with diabetes are prone to damage to blood vessels. This can lead to eye problems such as retinal damage, cataract formation, or vision loss. Diabetic cats can develop heart disease and kidney failure. Get your vet to examine your cat’s retinas at every visit, and monitor blood pressure if possible.

If a cat has had long-term elevated glucose levels, they can suffer nerve damage. Be vigilant for signs of weakness in your cat's hind legs. Sometimes it's more obvious as their hind legs slip out from under them with diabetic neuropathy. Diabetic cats with neuropathy walk down flatter on their hocks, meaning unlike other cats, they don't walk up on the toes.

There are pictures of this "plantigrade stance" here. Scroll 1/2 way down:
http://www.uspharmacist.com/index.asp?show=article&page=8_1000.htm#FIGURE2

Cats with diabetic neuropathy benefit from B12 supplementation. Make sure it’s methylcobalamin form. B12 injections are only available with a prescription in the U.S. and are cyanocobalamin form, but sometimes that’s the only way to reliably get B12 into a kitty cat.

For cats who do not have tummy issues (unlikely given the etiology of diabetes in cats), one can use oral B12 e.g. NOW brand. However, cats with IBD or pancreatitis or small intestinal disease have trouble absorbing B12 from pills. In their case, try sublingual methylcobalamin. Natural Factors brand is the only one without sugar, colorings, and other icky ingredients. You will have to pop in a pill, and hold your cat’s mouth closed for several seconds till it dissolves. Get falconer’s gloves or something first!

Dietary Modifications 

• Feed your cat small frequent meat-based meals; all cats need this but it’s critical for diabetic cats.
• Make sure his/her diet has no simple carbohydrates which are converted into sugar very quickly in the body. Be vigilant about so-called low-carb foods which may not have grains, but contain potatoes, fruits, and other such high-glycemic items.
• Diabetic cats sometimes need a little bit more moderately fermentable soluble fiber in their diet than their non-diabetic counterparts. 1/8 teaspoon of either rice bran (NOW and EnerG brands) OR 1/8 teaspoon psyllium husk mixed with water are good choices. If your cat won’t accept either of these, low-glycemic vegetables 5% or less of overall diet works well too.
• Keep honey on hand if kitty becomes hypoglycemic or starts shaking. Rub some along gums to bring blood sugar levels up.

Treatments 

Sometimes switching a cat (gradually, to be safe) to a raw or even grainless canned diet brings blood glucose levels back down enough to where the cat doesn’t even need insulin. I have seen this personally and on the list many times. If blood glucose levels do not come down with proper diet, treatment consists primarily of insulin injections.

Other times, one can cut back on insulin injections (with guidance from a vet, of course) by monitoring blood glucose levels at home as well as keeping an eye on urine output, water intake, etc. If your cat needs insulin, you must test BG levels before giving a shot of insulin. Do not depend on urine dipsticks because by the time glucose spillage occurs in urine, the BG levels can be quite high. In cats, the renal threshold for glucose is 220 mg/dl, far too high for the usual diabetic cat treatment range of 100 – 200 mg/dl.

Injection site

The injection site for insulin is extremely important, as it shouldn't get into the bloodstream too quickly (can cause death due to overdose), or get into a fatty area from where it will not be absorbed fast enough.

According to this paper by Greco,D.S., Broussard,J.D. & Peterson,M.E. "Insulin therapy". Vet Clin N Amer (Small Anim Pract) 25, 677-689 (1995), just like with humans, where the insulin injection is given is extremely important because different parts of the body absorb insulin differently. The full-text is not available online, so I'll quote from my hard copy.

In cats, people will often give shots in the scruff of the neck like with subcutaneous (SQ) fluids. According to these authors, this isn't ideal because of "low blood flow and increased fibrosis caused by repeated injections".

Instead they recommend insulin shots be given "along the lateral abdomen and thorax" and should be "rotated daily in order to prevent fibrosis at an injection site".

Thorax is the chest cavity and with lateral abdomen I believe they mean along the sides of the abdomen. IIUC, one shouldn't inject right in the navel itself but around and out to the sides a bit is fine. In some cats, one can lay them on their backs, and as long as one avoids the nipples and navel, stomach can be a good place. In humans, maximum and most reliably consistent absorption of insulin is from injections in the stomach.

Alternatively, if a cat will lie on his/her side, one can inject anywhere along the sides of the body including hip areas. Stay away from extremities because insulin is poorly absorbed from there due to movement.

And in that paper, they mentioned dorsal area (back), so if kitty is standing or sitting on his/her haunches, then moving out from the spine out to the sides like hips/flanks is a good place to give insulin too.

Choose different spots each day as recommended in the literature to prevent granuloma from forming. If your kitty is sore from a granuloma and/or from ear-testing for BG, apply pure emu oil alone or mixed with St. John's Wort/Hypericum herbal/homeopathic tincture.