Best place where I learned was over at
www.spotinjections.com, but i'll post some helpful hints here which I found better at times.
Shoulders - If you go too high up the humerous you'll hit your humeral nerve. Your insertion of the pin should be down toward where the delt inserts into the humerous, about a 1/4 the way down the arm, maybe just an inch or so higher.
Quads - This is the hardest to be honest. If you flex your leg you should see your vastus lateralis (and if you can't, then you obviously dont have enough muscle on you, thus shouldn't even be touching steroids). Follow the ridge up about 1-2 inches past half way, then go "down" lengthwise an inch. Hit it perpindicular. The same applies to the left quad, HOWEVER, WATCH out for your femoral nerve as this is the one everybody hits on the way in because they are TOO low when injecting into the leg. Too low meaning toward the middle of the quad in terms of the thick part of the leg. It must be up higher, and you must hit it perpindicular into the vastus lateralis, otherwise you severly risk hitting it (and most of us, including myself, know how much it hurts).
Glutes - no brainer. You have 4 quadrants, hit the outter most one. There is plenty of meat to hit there so dont try to be a fool picking another spot, lest you want to paralyze yourself hitting the sciatic nerve.
I'd go into the others, but they are the main ones people like to hit. I've yet to do bicep injections, and I will be now that i'm lean enough as I was scared before as i'm quite a vascular guy, so i'll be following the spot injection guide along with my own knowledge of human anatomy (in which i'm an exercise physiologist). But i'd always love to see videos of peeps doing bicep (and tricep) injections...anybody?!?!