Seems like you're living in a pronounced pelvic tilt and have a tight "sticky" low/mid back?
If so, my suggestions would be the already mentioned glute and core work (specifically anti-extension exercises). And, try rockback breathing: http://www.youtube.com/watch?v=08h0NCCYRj8
The emphasis is placed on breathing through your stomach and expanding your stomach and low back all 360 degrees. Deep breaths in and a FULL exhale out.
If you don't see almost immediate benefits, then I'd recommend going to a see a QUALIFIED physical therapist.
EDIT: I'd add in if it is an anterior pelvic tilt, the last thing you would ever want to do is stretch the hamstrings/glutes. These muscle groups are already on stretch 24/7/365 and would only feed into the problem potentially making it worse.