(Forget to add this--the TV shows work better than movies because most shows are 40-45 minutes, which is about how long your workouts should be. Alias is a great show to workout too, by the way. I had never seen it before but told myself that I could only watch it when I was also working out. Some weeks I even worked out six days a week just to watch a new episode! Good luck finding something that motivating!) :D
Invest in new mp3s or dvds to watch. Seriously, the only way I get motivated to work out is to buy entire seasons/series of TV shows that I've been wanting to watch for awhile. It's much easier for me to keep working out if I have something fun to watch, and my friend is motivated to run by good music. It seems like if you can create an atmosphere where you forget you're working out, it works better, at least in my experience. I hope it'll work for you too!