I have has physio for a back problem I had since pregnancy. The physio said my abdominal muscles have 'gone on holiday and not returned'!! So I know how you are feeling.
She told me to imagine that I had a balloon tied to my head, pulling me up. This causes the rib cage to lift and your tummy to pull in. Also when in this position, pull your tummy in a further quarter of an inch - all the time, when you're walking and everything.
I have been doing this just over a week noiw and already I can tell my abdominal muscles are feeling a bit tighter.
Good luck!