srb wrote:BJs and fresh baked cookies.
Worthless, witless response to a legitimate question.
As many people have suggested, there is no magic formula (and I don't know why "it depends on the school", unless certain posters were implying firsthand knowledge of select schools' waitlist histories). If feasible, schedule a school visit. Sit in on a class. And, while there, attempt (at least) to sit down with an admissions counselor. I've found it relatively easy, however, to schedule meetings with admissions deans and assistant deans (Pitt, Penn State, Seton Hall, Duquesne, etc.), given that you make yourself available for a range of dates/times. The only wall I've hit has been at UVA (where I was told that the deans won't schedule meetings ("interviews") with waitlisted candidates (though they did say I may be able to catch one during whatever day I choose to visit...take that at face-value. No guarantees). Post-visit write a good LOCI that relates your visitation experience/your underlying, significant interest in the school...in other words, make it personal/tangible. Also, definitely include anything important you've undertaken (project/thesis, work experience, etc.) since you first applied.
Apart from the above (and sending normal "extras" like updated transcripts, LOR's, etc.), there's not much else to be done.