Exercises that work the upper back. I like band pull-aparts.

I stopped carrying a backpack everywhere I went. Instead I put my computer in a much smaller messenger bag when I need to, and I switch which shoulder it goes over.

With the backpack, I had all sorts of shit in there just because one day I might need it. The backpack was constantly training my body to hunch forward.

Someone at work taught me this little trick for better posture:

Lift your shoulders as far up as you can.

Pull them back as far as you can.

Relax them back down, and try not to let them slide back forward as you walk/stand.

Great for a sore back.

You can get a strapped thing that pulls your shoulders back and forces good posture.