Keep a running track of your bills by using index cards. Office productivity blog Web Worker Daily has more:
Create a card for each bill, writing the name of the bill at the top. When you pay the bill, create an entry on the card. This way, you have a running log of all the bills you pay.
Very low tech, but effective. You can also use these nifty finance index card templates to make your index card organisation go a little more smoothly.