Do slow relaxed circles with your ankles. Stand on one foot with the other slightly in front of you off the ground, do nice relaxed circles, take at least 5 seconds for each rep. Do about 5 reps at a time, a few times a day.

My favorite: Stand with your feet about shoulder width apart, toes pointing straight forward. Either with your own balance or using a wall/chair back to steady yourself, raise up onto your toes for a count of three, then back down for a count of three (no cheating and dropping down fast). Do this ten times, then shift your feet so your toes are angled out and repeat. Then shift so your toes are angled in, and repeat.

I do this whenever I had to stand in one spot, like in line, at the sink while doing dishes, or at the copier at work.