I use an Excel spreadsheet and keep track of the date, mileage, and work done--1 tab per car. At the bottom of the sheet I recap all of the maintenance items so that you can enter the current mileage and it will show how many miles you have on a particular service (i.e. oil change, trans fluid, PS fluid, rotate tires, etc.) and how many more miles until you need that service again. I have all of the maintenance services (about 20 of them) in a vertical column so I can instantly tell what maintenance item is upcoming.