Limit the number of registrants allowed on the waitlist
We often have events that have large waitlists (25 people on the waitlist for a 20-person event). Rarely do we receive more than three cancellations, so the rest of the people on the waitlist have no reasonable expectation that they'll get in.
Stephen Metheringham commented
Excellent idea, plus ability to do this on each and every waitlist as we may have insufficient coaches available for certain periods of the year