I like Steve's answer. And not because I'm a kiss-a__...
Most people try to shift behaviors by attacking the problem. They ask themselves, "Why am I doing this? What's wrong with me? What do I need to fix?"
This keeps you focused on the problem. Which, most times, is a pretty miserable way to go about it.
Instead, focus on where you want to go. Visualize the positive future you're trying to get to, and ask, "What will it take to get me there? What positive qualities/habits/actions/beliefs will get me to where I want to be?"
This keeps you focused on the solution, which is a lot more enjoyable.
(It's a lot like being in a skidding car... if you focus on the tree, you'll most likely hit it. Focus on the open road, and you're more likely to find yourself there.)
If you're looking for more structured help, the approach of Appreciative Inquiry is a good starting place... a quick Google will yield some good resources to check out.