A friend of mine had this problem so bad when we practiced that me, the bassist, and the drummer would skip beats due to laughing sometimes. What we did was borrow a friend's camcorder and when he closed his eyes and seemed to be getting sexual pleasure from his lead we recorded it and showed him later. He got self-conscious about it and after a while quit. If you do it, record yourself and watch it... you'll get embarassed and (hopefully) stop.