If an injection seems especially painful or if blood or clear fluid is seen after withdrawing the needle, the patient should apply pressure for 5–8 s without rubbing. Blood glucose monitoring should be done more frequently on a day when this occurs. If the patient suspects that a significant portion of the insulin dose was not administered, blood glucose should be checked within a few hours of the injection. If bruising, soreness, welts, redness, or pain occur at the injection site, the patient’s injection technique should be reviewed by a physician or diabetes educator. Painful injections may be minimized by the following:
Injecting insulin at room temperature.
Making sure no air bubbles remain in the syringe before injection.
Waiting until topical alcohol (if used) has evaporated completely before injection.
Keeping muscles in the injection area relaxed, not tense, when injecting.
Penetrating the skin quickly.
Not changing direction of the needle during insertion or withdrawal.
Not reusing needles.
Insulin may be injected into the subcutaneous tissue of the upper arm and the anterior and lateral aspects of the thigh, buttocks, and abdomen (with the exception of a circle with a 2-inch radius around the navel). Intramuscular injection is not recommended for routine injections. Rotation of the injection site is important to prevent lipohypertrophy or lipoatrophy. Rotating within one area is recommended (e.g., rotating injections systematically within the abdomen) rather than rotating to a different area with each injection. This practice may decrease variability in absorption from day to day. Site selection should take into consideration the variable absorption between sites. The abdomen has the fastest rate of absorption, followed by the arms, thighs, and buttocks. Exercise increases the rate of absorption from injection sites, probably by increasing blood flow to the skin and perhaps also by local actions. Areas of lipohypertrophy usually show slower absorption. The rate of absorption also differs between subcutaneous and intramuscular sites. The latter is faster and, although not recommended for routine use, can be given under other circumstances (e.g., diabetic ketoacidosis or dehydration).
American Diabetes Association
Diabetes Care. Insulin Administration. 2002 Jan; 25(suppl 1): s112-s115.
Apart from this, I can only tell you that I've been predominantly taught to do subcutaneous injections with either insulin or heparin in the abdomen, though with no particular reasoning.