As you play your guitar, dirt and oil from your hands gets mashed into the fingerboard. In fact, the fingerboard is the dirtiest part of most guitars. When you’re changing strings, you have an opportunity to wipe down the fingerboard and really clean things up. (By the way, it doesn’t hurt any guitar to take all the strings off at once.) If the dirt buildup is really bad, simply wiping with a damp cloth won’t suffice. Get some extra-fine steel wool (grade #0000) and scrub the fingerboard gently across its surface, parallel to the frets. There’s usually no need for cleaning agents, because the steel wool will scrape the dirt away.