Do not try to consciously correct your posture. You don't know enough. Some evidence-- I tried it, and just gave myself backaches. I know other people who tried to correct their posture, and the results didn't seem to be a long run improvement.

Edited to add: I didn't mean that you personally don't know enough to correct your posture consciously, I meant that no one does. Bodies' ability to organize themselves well for movement is an ancient ability which involves fast, subtle changes to a complex system. It's not the kind of thing that your conscious mind is good at-- it's an ability that your body (including your brain) shares with small children and a lot of not-particularly-bright animals.

From A Tai Chi Imagery Workbook by Mellish:

Conscious muscular effort to straighten the spine, or alter its shape in some obvious way, generally recruits the long muscles on either side of the spine (the erector spinalis group). These muscles are strong, but because they run almost the whole length of the spine, they exercise only a very coarse control over its carriage.

He goes on to explain that the muscles which are appropriate for supporting and moving the spine are the multifidi, small muscles which only span one to three vertebrae, and aren't very available for direct conscious control.

A lot of back problems are the result of weak (too much support from larger muscles) or ignored (too little movement) multifidi.

He recommends working with various images, but says that the technique is to keep images in mind without actively trying to straighten your spine.