I don't have any furniture. I sit on the floor, and sleep on a Futon (Japanese-style http://www.yumetai.co.jp/img/goods/L/24623s1.jpg), which I can fold up and put away during the day.

Besides that, I just try and not collect possessions. If I don't use something, I give or throw it away. I avoid buying stuff that will take up space unless I regularly use it.