i keep everything in my log. date i brewed, ingredients list, original gravity, i track my visible activity in the ferm locks, temp of room i'm fermenting in, when i racked from vessel to another, taste, color, etc. that way when i brew a batch i've brewed before, i can compare and see why one may have turned out different than the other (which they always do seems like!).
A software package like Suds makes keeping a log easier, since it calculates a lot of stuff for you and gives you appropriate places to enter the various bits of info. I'd be hopeless in terms of logging if I didn't have Suds.