Per the Bicillin Label:
"Administer by DEEP INTRAMUSCULAR INJECTION in the upper, outer quadrant of the buttock. In neonates, infants and small children, the midlateral aspect of the thigh may be preferable. When doses are repeated, vary the injection site. Because of the high concentration of suspended material in this product, the needle may be blocked if the injection is not made at a slow, steady rate."
Hope this helps... although anecdotal nurses on my floor usually give it fast, too.