twice. had the same date both times. both of us went to single-sex schools, which fortunately had their proms on different days.
her prom was a total drag, though. just a bunch of bitchy girls... my prom was pretty fun. lots of guys went without dates, and it was a much more relaxed atmosphere.
the one thing I noticed at my gf's prom was that the girls were really into it and stressed out over the whole thing. I guess since it wasn't really their prom, the girls at my prom were a lot less stressed out and were able to have fun.