In this article, well look at what makes a good cover up, and your options for getting your hands on one.
You want the cover up to open all the way down the front for ease getting on stage. You might find yourself hurrying to the stage, or needing to step out of the audience and suddenly be in costume, depending on the venue, and you don't want to be fighting your cover up over your head potentially messing up your hair and makeup on the way. It should close all the way, not letting any of your costume show in the middle and not counting on you to hold it shut.
Velcro might seem like a good idea, it's fast and easy, but you run the risk of it getting stuck on your costume. Zipper fronts that separate, like on a hoodie, can work but you need one long enough and have to be careful not to catch your costume when zipping it on. Ties are ok but I find them problematic if you need to get out of the robe in a hurry and they can tangle. My cover up uses snaps and I'm happy with it.
Consider the neckline as well. Your cover up should fully hide your bra and at least some of your cleavage. Remember, the goal is to not draw attention from the other performers. More than that, you don't want to spill something down your girls and into your costume.
On the topic of not drawing attention, there are many beautiful colors and it's certainly normal to pick something light or even bright. You won't be called out for this, but if you're going to be backstage I recommend black. It helps you hide, as Ranya says: there's a reason stage hands wear it.
Lastly, pick something that is machine washable. Costumes need enough maintenance, don't add to your workload. Also, Pick something breathable. You'll be hot when you get off stage and won't want to wrap yourself up in velvet while you're dripping sweat.
Many costume retailers sell caftans and cover ups. They are usually very expensive and, while pretty, I don't usually find them worth the price. For less than most of those options, and for a better quality with perfect functionality, Ranya sells abayas imported from the gulf. I love these and someday will treat myself to one (after my student loans are beaten back a bit ^_~).
This is my first cover up. A bathing suit cover up maxi dress from j.c. penny. I cut it open down the front and added some pleated quilter's cotton to fill the middle to keep my skirts covered. I made a new one because it didn't cover the front well enough, without a scarfe (veil) worn down the middle. I keep it to lend to students. Robes of any sort would also work, provided you don't look like you're walking around the restaurant in a bathrobe.
This is my current cover up. I need to do something to raise the neckline and if I were going to do it again I would pick a fabric that doesn't wrinkle after being balled up in my gig bag. It makes me look like a federal judge, but I like it. I made it by modifying a pattern for a renaisance fair bodice that I had lying around. You can find patterns already intended to be some sort of robe at craft stores, in the costume section of the books or through bellydance pattern companies.