Make Posture a Priority.

Posture was something, when I was younger, that my parents would idly remind me of.  I quickly ignored it, and went back to playing Nintendo 64 in the same ridiculous contortion I had been in previously.  I then spent inordinate amount of time in front of a computer doing whatever felt comfortable.  This continued until I was 22 (to be fair I had worked on flexibility and a few other bits but my posture wasn't until this year).

Something similar to these two grainy obviously poached from Google Images kids

I fucked up.

This made working my 4 hour shifts at my first job horrible because my back would start aching.  It made learning how to lift difficult because I was plagued with awful habits.  It made my first couple shifts with the backpack vacuum cleaner slow and arduous.

Then I got smart.  Well...smarter.  I was already smart.  More to the point, I started working on my posture.

First off.  BENEFITS OF POSTURE:

1. A much more commanding presence.
2. You don't look like a slack ass good for nothing
3. You'll be preventing problems that will bite you in the ass when you get older.  No one wants to retire with a hump on their back.
4. Everyday chores and tasks are easier.
5. You will be able to ignore the PSA's that will undoubtedly arise in the next 20 years due to computer posture.
6. You won't throw your back out grabbing a bag of groceries.
7. Makes you look thinner (THAT'S RIGHT FOLKS)

How to fix your Posture:

The first step is to be aware, the trick to fixing something like this is to stay vigilant until it becomes habit.  Now I will provide two absolutely incredible links that will help diagnose and help you fix your posture.  I was going to summarize but that wouldn't do justice to the work these individuals have put in.  At the end of the day, I just want to see people improve.  Use these links and become thoroughly upright individuals!

70sbigs - This post is very comprehensive and will help you diagnose any problems you might have and go into much greater depth.

ArtofManliness - I would read this one second, a little more straight forward, and will help you with some strategies to improve.