Jeff Atwood has put together a convenient list of ways to spend money in ways that will make you happier (all backed up by science!). The advice is all golden, and summarized well:

3. Buy many small pleasures instead of few big ones

Because we adapt so readily to change, the most effective use of your money is to bring frequent change, not just “big bang” changes that you will quickly grow acclimated to. Break up large purchases, when possible, into smaller ones over time so that you can savor the entire experience. When it comes to happiness, frequency is more important than intensity. Embrace the idea that lots of small, pleasurable purchases are actually more effective than a single giant one.

Atwood references a paper co-authored by Dan Gilbert, whose excellent Stumbling on Happiness is a real eye-opener on human perception, emotion, and happiness. Highly recommended.