ill give you a tip that i use everytime i do it. i am unable to shoot in the shoulders anymore becuase of the huge buildup of scar tissue in there so quads are the only choice for me. (i hate the butt)
sit on the edge of the couch or the toilet. couch is better becuase it doesnt move on you.
anyway, sit on the edge of the couch and straighten your legs out as far as you can get them in front of you. now de-tense the muscles. with the legs straightened that way it should be very difficult for you to tightly tense the muscle.
ok, once you have your legs all the way out now locate where you plan on shooting. hold the syringe like a dart and line it up back and forth a few times measuring the distance. then on the last motion back and forth take a deep breath and right as you start to let out your breath poke it into the thigh. releasing your air will help to loosen the muscle as well. once you have the pin in just take a moment to relax. pull back on the stopper to make sure no blood.
if no blood then slowly push the fill. if you can distract yourself try not to look. i have a routine of watching Jay Cutler training on youtube while i do mine and i visualize myself getting stronger and bigger. makes the process more fun.....