It's great fun, but it's important to do your homework, first: A thorough enema, a wise choice of venue (assuming you don't want your bedroom smelling like a urinal as there is always the possibility of seepage) and knowlege of your partner's HIV status. (If you're neg and you don't know your partner's status or don't trust his status, this is not the safest activity for you, since you have to dispense with condoms to do it.)
If your still gungho about it, use common sense and have your top "clear the tube" (of any semen or pre-seminal fluid) with a bit of regular pissing first before he pisses inside you. And then, no lingering.
Just as much fun and a good deal safer is having your partner(s) stay outside entirely and aim his stream (from some distance) against your anus...Incredibly for both of you and incredibly good when the ol' back door's been pounded open...