Here are some good instructions.

http://www.ehow.com/video_115944_fix-garage-door.html

You can shorten the cables (same amount on both springs) to get more pull. I like the door to be in balance about half-way up or at chest height.
If the springs are 10 years or so old, you might want to get new springs (Home Depot, Lowes, etc) to replace the current ones. You'll need to know the weight of the door (using bathroom scales with the springs disconnected) in order to get the proper size replacements.