The best plan is to not sit for too long at one time. Get up and stretch and walk around at least every hour. And make sure you strengthen and stretch the muscles that get out of balance from too much sitting. The trainers at the Plex can probably help set up a program. But enough preventive solutions; you need immediate solutions, right? Try one of these if you’ve stiffened up. When you stand up, first do a few stretches–a quad stretch, a hip flexor stretch. Or try some bouncing and shaking: stand up (shoulder width stance), starting from the bottom of your feet, gently shake up and down for a minute or so, then make your motions smaller and smaller until you are still again. Good luck with finals!