Set up a double date with one of your good friends and a couple ladies. Once you are ready you, tell your friend to come on over and have a drink before we go out just to loosen up and get out any possible nerves that could over take the night. Ask him what he wants in advance that way you know to get him his own bottle.
Now you take out however much you need and replace it with a clear potent ex-lax. Make sure you call up before he gets there and seem as though you are more than excited for him to be coming over because you REALLY want to start drinking and you prep his drink in advance for the optimum dosage. Now after the first drink you want to set him up with one shot and then immediately go out so you can get out into the public eye before his brown eye decides to come out and play.
NOW A POSSIBILITY AND A PRANK ON A PRANK! (and this is only if fate is a fine sensual whore of sorts)
After your friend completely embarrasses himself and shits his pants as planned, The girl he is with sympathizes and he winds up getting laid anyways dumpy pants and all. At this point the prank is completely upon the girl!!