A simple log book or Excel sheet. Keep/scan the receipts and throw into an envelope. No need to write down all the details already described on the receipt.
I imagine there are numerous reasons to explain the different levels of record keeping. I bet the majority of non-BITOGERs keep minimal records. For OCD types, it is part of their pleasure/illness that drives them. There is a difference for selling an enthusiasts type vehicle vs. an utility/appliance type vehicle. I'm the latter type and people trust when I tell them about the regular things like brakes, filters, oil, minor repairs, etc.. I don't give them my personal vehicle records and I probably leave out unnecessary details. I.E., I would not tell them that this particular model (at 80K) has a history of leaking head gaskets after 100,000 miles. I don't track cost of ownership and like Kestas, only occasionally check mpg.