One good thing to do is start sucking your stomach in (especially good for people that have lower back probs too)

at first it's hard to maintain and really try and feel the muscles low down, after a few days you'll find you can hold it for ages. It is hard but after a while you'll find your core held together better and you can build a good posture on a good core strength.

I'd also advise situps and get into your head your core muscles pulling in and holding you tight.

It sounds a pain but trust me after a few days (you will feel a burn from holding yourself in at first) you'll get addicted to it. Before you know it your muscles will have pulled tight and your jeans will be hanging off. Keep situps up and build your posture on your new core strength.