Ways to keep in shape with a depressed metabolism?

When I was younger I was naturally thin, which seems to run in my family. I used to have such a high metabolism that allowed me to eat huge amounts of chocolate and other fattening things without worrying about gaining weight.

However, a few years ago I started taking a medication that has apparently changed all that. I’ve slowly been getting bigger and although I don’t consider myself overweight, I’m not happy with how I look.

I try to eat pretty well, although I’m not particularly active (being a double-major art student will do that for you). But even last summer when I rode my bike for an hour 4–5 days a week and stayed away from junk food, I didn’t lose any weight.

Have any of you had problems with a metabolism slowed by medication? Does anyone have any suggestions to stay in shape and compensate for a depressed metabolism?