I know what you mean, I sit at work all day as well, and there are no stairs. The office is also quite small, so I feel a little awkward if I'm getting up but not going anywhere. There is a water cooler that is across the office from me, and I make it a point to use a small glass, drink lots of water, and make frequent trips to refill. I also have a very bad

coffee habit, and after my coffee I find I get somewhat jittery and that's enough for me to find excuses to get up. I try to also find tasks where I can stand performing them(difficult when most of my job involves a computer on a low to the ground desk) or even get up and stand every now and then, because standing at least is better than sitting. Hope that helps, and good luck with it.