R.K.
Pilates is great for just that. Here's a wonderful video that I swear by:
http://www.amazon.com/Hilary-Burnetts-Pilates-Basics-Burn...
Before I got pregnant, I was in the most excellent shape in my life (at age 40!) and it was in great part due to walking regularly and doing this video. I know that it helped my back pain, etc. before and during and after pregnancy.
I've resumed doing the video (my baby is 14 months) and even with doing it only a few times a week, I can see almost immediate improvements to my core muscles. What I like about this video is that Burnett is a really good teacher, very encouraging, and the exercise is gentle and graceful, and the whole thing takes only about 20 minutes. Enjoy!