10 minute workouts
I had a similar problem when I was working during the day and taking classes at night, until I found exercisetv. On their website they have a bunch of free 10 minute workouts that I was able to fit in before work, after work, and before bed (yoga). It's so much easier to get in 30 minutes when it's broken up. Granted, it's not as effective, but it is better than not doing anything!!