For many years my Rules have stood me well when it comes to not buying too much wargaming stuff that I won't get around to using. They are:
1. 20mm is skirmish scale. If you do 20mm gaming it's with individually based figures in skirmish games. If you do skirmish gaming then you do it in 20mm.
2. 6mm is massed battle scale. If you massed battles, do them in 6mm scale.
3. Rules 1 and 2 are waived when building Hordes of the Things armies.
These rules, as I say, have helped keep me focussed. They are, of course, very personal. However, I've already broken them pretty solidly of late by rebasing all of my WOTR stuff for Impetus and now I'm weakening further.
I've been reading reviews of Black Powder and thinking "that sounds like the kind of game I'd like".
I also have loads of unpainted plastic Napoleonics lying around and this morning I found myself putting them on 40x40mm bases and going, "Hmmmm...".