Addiction is a disease. I find it helps a little to enjoy all aspects of my book. Don't just purchase the book and put it away.
Take pictures. Post them in your collection thread. Put on a Brodart jacket protector. Peruse or even, here's a big one, read the book. If you read every book you purchase before buying the next one it's sure to slow your collecting pace.
Make a game of setting certain restrictions on your purchases such as it has to be half price. That worked for me for a while...
Until Cemetery Dance had a 1/2 price sale.