I use a paint brush and compressed air with each usage. I make an honest effort to keep my mill as clean as possible, but I'll concede that it's about as clean as a snot-smeared, Goldfish Cracker-caked toddler. No matter how thorough I am, it always leaves a sprinkling of very fine flour whenever I set it down.
I sometimes wonder how much of the house character of my brewery is attributable to my impossible to really clean mill? Is my mill clean? Yeah. But, let's be real, there's no way in hell that I'm getting all of the flour out. Writing from the humid Mid-Atlantic, although there are no obvious outward signs, there's certainly some biology taking place within the deepest recesses of my mill. Is that really a primary driver of my house character? *Snort!* Of course not! But it would be foolish to think that something isn't going on inside there.
Anyway, I sometimes ponder this deep issue while milling at 4.45am on a Saturday morning.