How do you tell a friend that you want to have sex/be friends with benefits without being a complete prick?

I've always wondered how to go about this. How does one request that you become sexually involved with a friend and STAY friends or start an FWB friendship without sounding like you're just trying to take advantage of them? I know this is a little different for us, because we're all guys, but still. I know some guys who would be insulted by this. What's your approach to this matter?