The easiest way, of course, is to just keep a small notebook and pen in your glovebox. But if you've abandoned such ancient technology for more modern record-keeping, maybe you should give My Milemarker a try. It's a new website that helps you keep track of things like how much fuel you're burning, how much money you're spending on that fuel and how often you're changing your oil.
It's simple, ad-free interface even lets you text message your information from your mobile phone. Or, if you have a laptop (or an iPhone), you can input the info directly. It will even keep track of multiple cars.
It's a sweet melding of geek and automotive. Oh, and it's free.