I like data, so I keep absurdly accurate records. I don't necessarily use that data for preventive maintenance, but I have it. The one exception is Crank Brothers pedals. When they get over 2000 miles I buy a rebuild kit because I know I'm going to need it.

Anyway, the way I track my data is that I have a spreadsheet with all of my bikes and the components on each that I'm interesting in tracking mileage for. Then when I've ridden a bike, I'll update its entries in the spreadsheet. This can be as simple as you like. I will generally record tenths of a mile because my online route planner tell me that's how far I rode. That's clearly a significant digit violation (more precision than accuracy) but I'm OK with that. If you're just doing this for preventative maintenance it's probably sufficient to say, "Eh, I rode about 20 miles today." Even if you're off by 100 miles out of every 1000 that you ride, that's probably within the margin of error for when things are going to wear out.