My two cents: if the topic turns to relationships/romance, mention a same-sex crush/romance/whatever. If somebody freaks out because "I didn't know you were gay!!!1!", say "I'm not, I'm bi" and drop it.
Alternatively, you could sit everyone down, get up in front of them, and say " guys, I'm straight. STRAIGHT UP BISEXUAAAAAL!!!" and run out of the room before anyone can react.