deadlifts help and also stretching the chest and anterior shoulders along with strengthening you mid/upper back.
Also, visits to your Chiropractor will help keep the spine lined up to help keep you straight.
Hope this helps.
Oh yeah, I wouldn't wear the belt unless you are going very heavy. If you wear it all the time you will weaken your low back muscles.
To fix posture you'll need to first determine the problem and then decide on the corrective action/exercises.
eg. A lot of guys in the gym have a shoulders forward posture which comes from too many chest exercises without enough back work and underdeveloped rear delts. The solution to that particular problem would seem obvious.
Too much ab work without any lower back work can cause postural disturbances too and visa vera. Balance is the key.
But wieghts and not the final soulution. Stretching works, yoga moves are probably the best. The best cure I found was simply standing flat against a wall with feet, calves, glute, shoulders and head toutching for 15 minutes a day,. You will notice the results immediately