Get in the habit of smiling. Smile when you are greeting someone, smile at the cashier when they ring up your groceries, smile to yourself when you are alone.

As far as I understand it, the physical act of smiling (for whatever reason) improves your mood. Personally, I've tried to make it a point to smile whenever it occurs to me, and I've found that it generally improves my day. In particular, I find myself feeling more positive and optimistic.