you mean the elastic slip up from under the skirt's (or pants or whatnot) elastic?

i do 2 things:
-wear underwear either skin tone or a similar color to the costume
-wear low-rise underwear (or if you don't have any, you can fold over the underwear's elastic once or twice. and if you want, pin the costume to the folded over elastic...will help hold ;)

hope that helps ;)

Or... you could be scandalous and not wear any! :O

Or you could even decorate your undies on top to match! Just one more layer?

I firmly believe in proper fit and color. I wear Victoria's Secret low-cut bikinis, they fit me very well. And, I religiously match my underwear to my costume. Remove tags, too! So, even if something goes wrong, chances are pretty good that nobody ever notices - and my underwear could almost pass as a bikini even in the worst case.

White underwear under a costume is not the best choice, unless of course the costume is white ;-)

I love low-rise boy cut underwear because it doesn't stick up and it doesn't ride up in the back (if you know what I'm getting at) while you dance. Maybe get some underwear that matches your pants/skirt too? If you have black pants and black underwear, it wouldn't be an issue, right?

Victoria's secret sells awesome underwear that has no tags and is made of the spanxy time material so you dont get panty-lines even if you're wearing full coverage ones! and they come in basic colors and crazy prints. And because of the material they're made in they stay pretty snug to your butt.
Other than that I agree with some of the others, if it keeps riding up you might just want to lose 'em :O You can also buy the little undershorts that cheerleaders and dancers wear and match it to your costume.

I wear low-rise bikini underwear and generally try to match it to the color of my pants/skirt/whatever I'm wearing. Haven't had an issue so far, even with all my low slung dance pants!

I dunno what kind of undies you're wearing, but you could try low-rise underwear of any sort, or like one girl mentioned, just try pinning your current underwear to your pants/skirt to keep it all in place. Matching colors always helps too! Your mileage may vary, but personally it would make me nervous to go commando, just because you never know when Murphy's Law will kick in and suddenly you find your bottoms around your ankles. Of course, I guess that would also apply if you pinned everything together.

I guess just play around and see what works for ya! Believe me, though, most people would rather see a bit of undie elastic than a thong string or bare flesh - that is, if they're the ones watching bellydance for the RIGHT reasons! ;)

All very good recomendations. Every one of them would work very well. My personal choice.....commando. All commando..... all the time.