Nearly all guitar fretboards are made from one of three kinds of wood--maple, rosewood or ebony--and each requires a different approach to cleaning. Maple is the most common fretboard wood and is sealed at the factory with lacquer or varnish, making it the easiest to clean. Rosewood and ebony fretboards are unfinished; the natural oil in the woods acts as a protectant. Cleaning your fretboard shouldn't take long if you already follow the musician's mantra to wipe down the guitar with a dry cloth after every time you play.
Remove the strings from your guitar. It's a good idea to clean the fretboard when you install a fresh set of strings.
Pour a small amount of lighter fluid on a toothbrush and scrub lightly around the frets to remove gunk and grime. Gently brush the wood between the frets.
Wipe away the dirt and repeat Step 2 if necessary.
Burnish the metal frets with a small piece of #0000 steel wool until the shine is restored. Take care not to rub on the fretboard wood.
Pour a small amount of lemon oil on a clean cloth and rub maple and ebony fretboards with the cloth. A small amount of lemon oil will do; there's no need to saturate the fretboard. Use tung oil for rosewood fretboards.
Wait a few hours to let the oil absorb into the freetboard before restringing your instrument.
Wipe down your guitar with a dry towel each time you are finished playing to reduce grime buildup from your fingertips and hands.
Keep yourself and the guitar away from heat sources and open flames (in other words, no smoking) while cleaning the instrument with lighter fluid and rubbing with oil.