Wear a tie. For prom, go with a bow tie instead of a long tie. Opt for a standard bow tie and, unless you are especially clumsy with your fingers, go for a tie that you have to tie yourself instead of using a clip-on version.
Regarding color, the safest bet is always black. If you know what your date's dress will look like, though, you might be able to pull off a tie in a coordinating color.