Remove all your strings from the guitar. It's best to do this when you need to change your stings anyway.
Step2
Using lemon oil (which I personally recommend) or mineral oil, put a small amount of oil on the cloth and apply to the fretboard. Make sure you rub all up and down the fret board.
Step3
Let the oil soak completely into the wood until it is dry. This is because you don't want to get it on your new strings as it will wear down the strings' integrity and create residue quicker.
Step4
Lastly you will restring the guitar up after the oil has COMPLETELY dried. Presto! You have a beautiful board once again! I recommend doing this once every 2-3 string changes depending on how quickly you go through strings.
Tips & Warnings
This works best for non-maple fretboards. (i.e. rosewood, ebony, and other various dark woods)
Try your best not to get this on the guitar's finish. While it won't necessarily ruin the finish, it isn't good for it.