I keep all chagnes to the car (maintenance or upgrades) in a text file in this kind of format:

5/2/03 104,586 Miles

Oil Change - Mobil 1 (Pennzoil)
RainX

6/28/03 108,659 Miles

Intake Manifold Overhaul:
Cleaned TB, UIM and LIM
Replaced head-LIM, LIM-UIM, TB, EGR, and IAC gaskets
Replaced injector O-rings
Replaced fuel pressure regulator
Replaced fuel rail to LIM seals
Cleaned MAF
Replaced Spark Plugs

7/3/03 109,086 Miles

Mounted new Yokohama ES100 summer tires on summer rims
Mounted summer wheels


I dont really keep maintenance receipts unless it was really pricy or out of the norm. Its nothing fancy at all, but its worked great so far.