2thepain has a good method. Other methods are depressing the skin with the needles and then turning the needle a bit, like a drill. It sounds painful but the needles is so small it doesn't hurt any more than normal, especially with a slin pin.
My favored method is to flex the bicep (not too intensely, just enough to make it solid), insert the needle (the skin won't stretch because the muscle is flexed), relax the bicep but don't extend the arm (because the muscle will move, and with the needle in it it won't feel pleasant), aspirate/inject. I do this same thing with my chest and triceps and it works just fine.