-Eliminate back and joint pains
-Improved breathing and energy
-Looking younger, taller, slimmer, more athletic, healthier.. sexier
Beware though, like all greatly beneficial things in life, good posture comes at the price of dedication, effort and time. Depending on starting point, it may involve:
-breaking a variety of bad habits, through diligent self checking/correction
-deep stretching of muscles/tendons that have become too short
-strengthening of musculature, especially in the back
-improvement of joint flexibility
-replacing furniture that invite bad posture
-reducing activities that put you in bad posture
But the first challenge is understanding what 'good posture' actually is, both logically and on a body/sensation level. Traditional advice like 'chin up, chest out' is not very helpful. Getting instruction by taking a ballet or yoga class, is without doubt the fastest way to develop such understanding. In 1-3 sessions, you could probably acquire understandings that could take months when trying on your own.
If determined to try on your own, hopefully my illustrations bellow can give you some idea.
You may want to read my guide to understanding the human spine.
There are also some good video resources on the net. Examples:
Yoga posture correction
After acquiring this understanding, and learning how to position your body, and what degree of tension or relaxedness is required in various areas, you are ready for the next challenge; Finding time to practice regularly. Without extended practicing several times pr week, improvement is not very likely.
Finally, if you don't take a class, checking your progress becomes vitally important, since you have no instructor to correct and provide feedback. A mirror simply won't do. You have to look at yourself from all angels without needing to twist around. Possible solutions:
-Video recorder on camera, phone, laptop web-cam etc.
-Have someone take pictures of you.
-Looking at a wall mirror through a hand held mirror.
Illustrations as promised:
Examples of stretches for enabling better posture