Here is an awesome info-graphic from MedicalBillingandCoding.org about sitting.
OH NO I SIT ALOT WHAT DO I DO AM I GOING TO DIE!??!?!
Bottom line: Get up and move around. Take breaks. Go pee, or the opposite, get a drink! Put your printer far away so you have to get up for it. Make a workstation like this: http://www.boingboing.net/2011/05/17/standing-desk-jockey.html and get a high bar stool for when you need to sit once in a while. DON’T get a comfortable chair. Go outside. Walk to work. Stop playing so many videogames.
UPDATE: maybe don’t make a workstation like I posted above. Here’s a good article showing how its -movement- that is important, and standing all day isn’t much better.