Work in upper back exercises too along with ab/lower back. They strengthen your whole upper body so your shoulders don't slouch forward.

Simplest and easiest way to build up your upper body muscles at home is pushups and pull ups. Get one of those pull up bars that attach to door frames. I have an "Iron Gym" one I got at Target for 20 bucks. No screws or anything needed. Pull ups will also strengthen your lower back and abs too. If you can't do a pull up just start with negatives (google it) and you will be able to do them in no time.

Join a gym, consult a personal trainer and get proper advice/training. They will have you focus on core exercises most likely, but much better than following fitness advice from a ragtag bunch of guys on a car forum..."join the military"...seriously? Bad posture can lead to some bad effects like scoliosis and chronic back pain. You might be able to get a referral from your PCP for physical therapy if the posture is connected to a condition. Much of what I've been reading in here is what would be considered "broscience". Get some good advice from a doctor or certified personal trainer on your next checkup. Gym memberships are also cheap, just don't set off the lunk alarm http://www.youtube.com/watch?v=umFtq_GOyxo&feature=related

There are a lot of dumb ass trainers too. Especially at the chain gyms. Not dissing that line of work, but I'm just sayin'. Be careful with advice, as with anything. It's better than the internet too. Umm... my uncle was in 'Nam and he has terrible posture. lol

http://www.youtube.com/watch?v=nrdFSWNs1xE&feature=related