I use a green Army notebook (thanks to the taxpayers for supplying this)...
It's just graph paper in a book-binder. Just note the mileage & service performed. I also save ALL receipts of purchased mods, parts, & service...These are invaluable when either selling parts or a bike...Even though you'll NEVER recover your purchased price on add-on parts, it's nice to know a vehicle that is meticulously documented.....you can also compare/recall service rates, length of time to perform a job, etc....This is useful when $$ is tight & service needed. You can also help out others by letting them know pricing averages, etc...
I don't document mileage...Unless I ride the EXACT same route every tank of gas, the mileage isn't the same & neither are the averages...Your gas mileage is dependent on an ENORMOUS amt of variables...weight (1 up/2 up), tankbag/tailpack...amt of gas in bike...weather ridin in (hot/cold/humid)...road surfaces...tire tread condition,oil condition, chain condition....just to name a few....
I understand that the avg shouldn't be TOO far apart/different, and MIGHT indicate a pattern of something wrong happening (underlying problem), but I have other things to do in life besides tasting oil/measuring tenths of millimeters...my bike is for riding, not measuring...after all, I'm not on a WSBK/MOTO GP team where microns matter......just my $ .02.......