I was wondering if anyone knew of any exercises or suggestions for correcting an overcurved lower back.

I am a woman, so it is not uncommon to have a slight curve in the small of my back, but mine is more distinct, and gets me in trouble for "letting my gut hang out". I've tried lying flat on the floor, and pushing the curve straight to get the feel for it, but it would be helpful to tone that area so that I wouldn't have to think so much about it while dancing. Do sit ups help that area? I just have no idea what to do, but until I figure something out, my posture will be all wrong! Thanks for your help...

Sure there, are exercises you could do - for example "the hundred" from pilates. But you could also just do a lot of solo practice someplace with mirrors, being really really self critical about the picture you are creating, carrying huge amounts of tensions in your abdomen until you can stay toned there while being actually relaxed in the rest of your body.