The following is from a textbook I'm reviewing for a course of mine this fall. I'll try and fill in some blanks afterwards.
"The gluteal region is a major site for administering intramuscular injections. When giving such injections, extreme care must be taken to avoid piercing the thick sciatic nerve that lies just deep to the gluteus maximus muscle, which innervates much of the lower limb. Furthermore, the needle must avoid the gluteal nerves and gluteal blood vessels, which also lie deep to the gluteus maximus.
To avoid harming these structures, the injections are most often applied to the gluteus medius (not maximus) muscle superior to the cheeks of the buttocks, in a safe area called the ventral gluteal site (Figure 30.6b). To locate this site, mentally draw a line laterally from the posterior superior iliac spine (dimple) to the greater trochanter; the injection would be given 5 cm (2 inches) superior to the midpoint of that line. Another safe way to locate the ventral gluteal site is to approach the lateral side of the patient’s left hip with your extended right hand (or the right hip with your left hand). Then, place your thumb on the anterior superior iliac spine and your index finger as far posteriorly on the iliac crest as it can reach. The heel of your hand comes to lie on the greater trochanter, and the needle is inserted in the angle of the V formed between your thumb and index finger about 4 cm (1.5 inches) inferior to the iliac crest."
Notes:
Posterior superior iliac spine (dimple): These are the two skin dimples everyone has at the base of their back, top of their but. They're there because your skin is attached, via fascia, to your hip bones there.
Greater trochanter: This is the large bony bump you can feel at the very top outside of your thigh. It's roughly at the same height as your penis.
Anterior superior iliac spine: This is the bony bump you can feel with your index and middle fingers when you place your hands on your hips. It's the very front of the crest of bone that runs from your posterior superior iliac spine (dimple) all the way around your to the front of your hips.