I keep an Excel sheet on each car, recording the date, mileage, and repair. If you're really clever you can scan receipts, warranty statements, etc. and embed them into the worksheet at the appropriate line. I also add reminder lines to myself highlighted in yellow ("will need new radiator in 20k miles").
Under the hood, I use a small Brother labelmaker to prepare tapes listing on two lines what I did ("Timing belt") and the date/mileage. Use alcohol to clean off an underhood surface and apply the tape label. It's a great way to keep an ongoing maintenance record right under the hood and remember what you did. With four Volvos, it is impossible to remember. It's also a good way to record fluid changes, etc.