1. Don't pick anything too suggestive.
2. Don't wear a push-up bra. Tuck those puppies in.
3. SPANX. These genius little briefs will save you multiple times from multiple problems. Trust me, paying five bucks for these is a solid investment.
I think the thing to remember in any case is to wear what you are ultimately comfortable with. If you feel tacky, then don't go that route. I have found that most Halloween superstores have good selections of cute and appropriately sexy costumes, but expect to pay around $50 plus more for the extras.
As for the men,
get creative! Don't be that typical guy that lets his girl do all the work with the costume. If she is going full out, then at least slap on some guyliner. This is the one night of the year when you can be crazy fabulous and go all night. Seriously, the sky is the limit, and you can think of some awesome and creative things!
As in everything with your personal style, be original!