I use an Excel spreadsheet
 

I've kept maintenance records since 1984. I keep up with the miles on the car at the time of the repair, the cost for the parts and a separate column for labor, the date the problem was noticed, when the actual repair was done, and exactly what was done.

The spreadsheet is separated into Planned and Unplanned maintenance. After all, it is the unplanned items you have no control over and that hit you the hardest. They are the ones in my mind that contribute to how reliable or unreliable a car is. Items that are discretionary are kept in the Planned section. When you look at only the Unplanned cost, the actual cost of ownership of my SC has been very low over the 21 years or so I've owned it. Actually, my Unplanned items only total about $4400 in 21 years. This includes things like a failed alternator, clutch replacement, etc.

Add in the Planned things like turbo tails, stiffer torsion bars, etc. and the total is about $19,000 in total over 21 years. Less than $1000 per year is not bad considering the improvements I've made to the body, suspension, and interior.