I have a small (about 2ftx1ft) shell that has all my "bought but not read" books. If it has no space on it, I do not allow myself to buy another book until I've read some to make space.

It hasn't really slowed down my bookbuying any, but it has sped up my bookreading, which is better anyway. I generally read in the morning before work, either on the bus (if I'm commuting) or in the coffeeshop nearest work (wherever that happens to be at the time). Just half an hour a day gets a lot of books read during the year.