1.  It can only last three months, max.  Then if you DO want to extend it, you have to talk about it first.

 2.  You can't tell your friends.  If they find out, they'll either give you crap, or act weird around you.  Either way they'll ruin it.

 3.  You can't go on dates, it has to be purely about hooking up.

 4.  No public displays of affection.

 5.  You can only get it on between 10:00 P.M. and 4:00 A.M.  The rest of the time, you're just friends.

 6.  You can only cuddle for 10 minutes afterward.

 7.  You have to leave before breakfast.

 8.  You can't leave anything at the other person's place, like clothes or an extra toothbrush.

 9.  You can't get jealous.

 10.  You have to agree that the friendship is the most important thing.

 11.  You have to agree you won't fall in love.  Which sounds obvious, but it has to at least be a ground rule.