I'm no expert, but I really think Yoga/Pilates has been helping me in that area. One exercise that I think really helps (because I can feel it) is:
Lay face down on the floor ( a yoga mat is really nice to stay comfy)
Put arms in a diamond shape and rest one hand on top of the other.
Pull your belly in toward your spine
Cross your legs one over top the other, almost mimicking the shape of your arms.
Lift your legs up off the floor and hold for 3 seconds, slowly release.
I repeat this 30 times and I honestly think I see results.