In my Dturbo I shoved the invoices in a folder. I did start a record on the computer but never kept it up.
Keeping track on when it's due I go off the service indicator on the dash.
I'm thinking of getting some of these to record things on the GTI.
http://www.ebay.co.uk/itm/10-x-Blank-Service-History-Book-Maintenance-Record-replacement-for-any-car-/191873903240?hash=item2cac936a88:gAUAAOxy86RSXWSC
They look good but I think I might need a bit more space for comments on additional work eg suspension work or aircon.
I might just print my own version off at work.