I will frequently set a 30 minute timer, to get up and move around a little bit. But if I'm really involved in something, I don't necessarily always get up when the timer rings.

One potential idea is - have ANOTHER computer where you put your guilty pleasure websites. (Block them from your main computer via the hosts file). Or even block useful websites, that you need to use regularly (email?).

Have that other computer plugged in somewhere without a chair. So, whenever you need to use the blocked sites, you have to move to the other computer, and be standing.