Most people that purchase art as an investment are focusing on high-end, blue chip art with a proven sales record.  As this article (The Art of Investing: The Rewards Aren't Always Financial) suggests, if you are buying art from an emerging artist, it is quite unlikely that you will be able to turn around the next day and sell that piece for more money. But they do make this point, which I could not agree with more:

Art is different than other investments. It's there in your house, part of your life.

"You are telling people something about yourself when you hang it," Moses says. "And therefore, I think that emotional investment gives you a certain tie to that work that you don't find in other objects that you buy."