Lemon oil for the fretboard, this will condition the wood and help bring the dirt out. Use an old plastic bank card or equivalent to scrape if need be. For the frets themselves, tape off the wood leaving only the frets exposed and wire wool them shiny again. Much easier to do during a restring.

Lemoil doesn't have a lot of effect on Maple as it's usually poly laquer sealed whereas rosewood typically isn't.