I know a lot of people who are busy at the office all day, and find it tough to fit in a healthy lifestyle, so here are a few small things that can help =)
Wear a watch and set it for every two hours, and take a ten minute WALKING break. (This is also a good time to have a snack, as you really should try to be eating every two and a half, three hours)
Really focus on using your leg muscles to sit and stand, don't just plop down.
Stand and pace during phone calls
Stand up every time you send an email
Take the stairs
Use hand gestures
Tighten your core muscles while sitting down. (Using an exercise ball as your seat will force you to do this, but it is possible if you just focus on it on a regular chair)