GUILTY!   I have 5 cars, here is what I do...
 
I buy a mini note book for each car.  I buy those Mead 4x6 inch "fat lil' notebooks".
Then I get an expanding accounting folder for each car and put it in my filing cabinet.  I start with having the purchase transaction and the title in the folder.  Also any documentation I got from the previous owner including the owners manuals and previous repairs.   
 
Everytime I do something to the car I enter it into the notebook along with the date and mileage.  That give me a historical record so I can refer back when the last time I did something.  (like oil changes, valve adjustments, plugs, etc)   This really helps as I put 500 miles a week on my 73 Pinto just driving to work.   
I throw the receipts for key items like tires, batteries, or restoration items into the folder.   I don't save all the misc receipts for oil, filters, etc. 
 
It is a very simple system and you can easily hand the notebook and folder to the next owner of the car if you sell it.   When I sell my 73 Pinto in the fall of 2012 I will have every detail since 22,000 miles for the next owner.  A local may want it, he wants the 1600 engine for his old Lotus 7.  He knows how good the engine internals are because of my records.
   
I also have an Excel file with a tab for each car (and boats) as a "TO DO" and "TO BUY" list.   This helps me stay organized on what I still must do or buy.  So I can plan what I must do next to what car.  It is not hard to manage since we only daily drive the 73 Pinto and the 84 Audi.  The other cars are "works in progress" until I have time or money.    Right now lacking both! 

What I'd like to do is get PEMR books for my cars. PEMR (if I remember correctly) stands for Preventative Equipment Maintenance and Repair. All equipment where I work has one. It's a sort of log book that you'd write repairs made, or preventative maintenance (oil changes, lubing, flushing, etc.), and parts replaced. It's also where we keep vehicle registration, accident cards, lube charts, and other incidentals. A book like that fits in the glove compartment and I can see at a glance when stuff is due or when something was repaired or replaced.