Thank you everyone, for all of the great suggestions! ^_^
I’ve worked with a lot of combs and brushes through the years... I used to style high-quality synthetic wigs a lot, and I quickly learned which methods were safe for maintaining them. This know-how has tranlated beautifully to caring for doll hair.
First off, save brushes as a last resort. Combs are much less damaging, and offer more control. Also, there are a whole lotta combs out there, and you should have various types on hand for different uses.
Personally, I have two steel combs (actually designed for dog grooming, lol), each with differently spaced teeth. These are handy for use in boil-straightening/de-frizzing, as the teeth are very rigid and don’t bend or soften. I also have a menagerie of plastic combs, some cheap and some higher end, in a multitude of sizes and makes. Other useful tools include flea combs, toothbrushes (great for fluffing plush!), hair-dye applicators, etc.
Cleaning synthetic hair is... not really my forte, though.
Unlike ponies and dolls, most theatrical wigs will never be played with in sandboxes, bathtubs, dusty playgrounds, and sticky tables, nor smeared with fingerpaints and markers.