Yes, pilates and yoga are helpful.

You should be able to tuck in your lower abs (think pulling in the lower belly like you're putting on a tight pair of jeans and don't want to get anything caught in the zipper) and still breath normally. If you can't breath normally, you're trying too hard. You should be able to do this and keep your glutes relaxed. It takes some practice. I usually have patients do this contraction, hold for 10 sec and repeat for 1 to 2 min twice per day. If you want to try to do it a little more you can try while walking etc, but don't force it.

As for stretches I like this one:

http://www.easyvigour.net.nz/fitness...xorstretch.htm

Hope that helps.

ETA: yes too many crunches can make this worse, especially if your not keeping your back flat against the ground while doing them and yes your posture and spinal curve can improve significantly with practice.