Ask for a desk you can use while standing. If your boss doesn't go for it, stand up to take phone calls or swap your chair for a stability ball.
Use the restroom farthest from your office, and drink extra water to stay hydrated and increase your number of trips to the loo.
Send your documents to a printer on another floor or on the other side of the office.
Create a screen saver that reminds you to stand up and stretch.
If you commute via bus or train, pace up and down the platform or sidewalk while you wait for your ride.
Do mundane tasks (bill paying, e-mailing) while standing.
Pace around the house while you talk on the phone.
Carry in your groceries one bag at a time.
Take a few minutes to make your bed every day.
Chop your own veggies instead of buying precut produce.