Students in my class are given two passes per marking period. Each marking period's pass is a different color. Students must present one of these passes if they need to use their locker, go to the restroom, or get a drink. Once the student has used all of his or her passes for the marking period, he or she may not leave the classroom unless a generous student is willing to donate one of his or her own. You'd be surprised how infrequently they need to leave!
Extra passes may be turned in for extra credit, at the teacher's discretion.