I think I'm posting this so someone will yell at me.
I get one of those double sided sponges with the mildly abrasive side, dampen it slightly, scrub the fretboard and quickly wipe any moisture off with a dry rag. When I'm done I use a little guitar polish. I only do this a couple times a year.
I used to take the neck off and soak it in a tub of hot water for a night, but I think that messed my basses up. They never played the same after that for some reason.