The abdomen, backs of the arm, thighs, hips and buttocks are all typical places that insulin may be injected.
Though these areas can absorb insulin differently, ease of injection has a lot to do with how successful your blood sugar maintenance will be.
The abdomen is the easiest place to reach on your body, and you can also see what you're doing more easily. Try to not to inject within 2" of your navel in all directions.
The thigh is the next in line in terms of easiest places to inject insulin. The back of the arms, hips or buttocks are more difficult to see and reach, and choosing them may make it more challenging to inject properly.
"Insulin Injection Sites." Health Information. 04 APR 2004. University of Wisconsin Hospitals and Clinics Authority. 27 Sep 2007.