Help with stretching at work...

I, like a lot of people, sit at a desk for 8 hours. I've noticed recently that my back has been hurting more than usual. I think I've pretty much verified that it is due to me leaning forward too much when I'm sitting down. I've decided I need to stop that and retrain myself to sit up straight.

Another thing I'd like to do is start stretching more often at work. So I set a reminder to pop up every hour so I can get some stretches in. My question is what are some quick stretches I can do at or near my desk. I figure about 5 minutes of stretching each hour would be good, right?