Are you worried about looking like the hunchback of Notre Dame? Here's a painfree and effective method for safely correcting posture and improving muscle tone.

Steps

Know what good posture is. Most people think that to "stand up straight" means flattening your back and ramming your head backward. Not so. The spine has a natural curve that you need to maintain. Proper alignment places your ears above your shoulders, above your hips. These points make a straight line, but the spine itself curves in a slight S.

Align your ears, shoulder, and hips. You'll find that this doesn't hurt at all. If you do experience pain, look at your sideview in a mirror to see if you're forcing your back into an unnatural position. If so, stop it! If not, see a chiropractor about getting your spine properly aligned.
Do exercises that strengthen the muscles across your upper back and shoulders. These do not have to be strenuous! Try the following, with or without handweights:

Align your ears over your shoulders. Raise both arms straight up, along side your ears. Remember to keep your ears aligned! Bend forearms toward shoulders to touch your shoulder blades. Do 10 repetitions with both arms, then alternate 10 reps for each arm singularly.

Align ears with shoulders. Raise both arms out to sides at shoulder length. Hold for a slow count of ten. Slowly lower arms to sides, counting ten as you lower. Slowly raise arms back to shoulder height, counting to ten as you raise arms. Do ten reps, constantly checking your alignment! If ten reps are too many to start, do as many as you can. You should at least feel a slight fatigue in the shoulder muscles.

Be a penguin. While you wait for a webpage to load, toast to pop, or the microwave to beep, place elbows at your side, and touch your shoulders with your hands. Keeping your hands on your shoulders, and your ears aligned, raise both elbows (count one, two) and lower back to your waist (count one, two). Do as many reps as your wait allows. You'll be surprised how much exercise fits into 30 seconds.
Do stretches. This can greatly help if you find that you have a sore back or neck after a while.

Tilt (stretch) your head in all four directions over your shoulders (forward, back, left, right), and gently massage your neck. Avoid rolling in a circle, as it may cause further strain.
On your hands and knees, curl your back upwards, like a cat, and then the opposite. Think about being able to place a bowl in the hollow of your back.
Repeat the exercises a few times each day. Doing them in the morning helps your body stretch out the muscle lethargy of sleep, and periodically throughout the day helps raise your energy level without a heavy workout.
Whenever you pass a reflective surface, take a sidewise look and check that your ears, shoulders, hips align.

More...

Don't tighten up your muscles when you are assuming a straight posture. It will only stress the joints and muscles themselves. Try to eventually relax into it, and you will find it will become easier and easier until it is instantaneous.

If your head is hanging, you can't be properly aligned. Keep your head at the level that allows you to look directly ahead without having to turn your eyes up.

A great side benefit of keeping your head straignt, and your ears/shoulders/hips aligned is an improvement in your self-esteem and attitude. If you walk with your head up, you appear more confident, and you feel more confident, which improves your attitude and mood, which makes it easier to walk with your head up.