There is no way to do this, it takes your sexual organs to clean themselves about a week or so, for sperm to be ejected from the vagina. However, this is not guaranteed. Pregnancy or STD contamination can still occur.
You've asked a medical question which we are unable to answer on Wikianswers. Take the advice of your medical practitioner; there are no medical practitioners here on Wikianswers, so you are best advised: Use a condom or another type of birth control when attempting sex or go and see your doctor if you already had unprotected sex.