Know who your teen is driving with at least a week before the prom. If she doesn't know, then be firm about being the one taking her. This will keep your teen from trying to pull the wool over your eyes at the last minute and drive with someone you don't trust. My daughter pulled this stunt and ended up in a small car with 8 other teens who were drinking.
Read what others have said and offer your advice and experiences: Prom Tips for Parents of Teens.