One thing that I recently started doing - after 18 years back in the Hobby - was to keep a paint diary.
I bought a $3 notebook down at the local bookshop and use it to keep a record of various paint recipes I use. I'm not sure why I never did this before as it is a pretty common trait among army painters.
The obvious benefit is that it lets you recreate the same colours at a later stage if you wish to add to or replicate a unit.
For instance with my terrain I've tried to keep the same unifying colour for the shingles on buildings. Now I have it copied down so that I don't have to guess as to the paints or choices.
It's a simple tool but one I think will make life easier in the long run.