A guitar should be thoroughly cleaned every couple of months to keep it looking its best, and, more importantly, to keep it in working order. Several different guitar cleaners and polishes are on the market (some of which work very well), but guitars can be cleaned quite well with common household items.
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Cleaning the Body and the Neck
In most cases you can use a damp cloth to properly clean the body, neck and headstock of your guitar. Some guitars with a veneer finish may accumulate greasy markings -- where your picking arm rests against the guitar body -- that are difficult to remove with just a damp cloth. A small amount of oil soap added to the damp cloth may help remove the accumulated grime on a guitar with a veneer finish. To clean your guitar as well as possible, you will want to remove the strings before cleaning the instrument. If the cloth you are using to clean your guitar becomes dirty, throw it in the hamper and find another clean cloth to use. Refrain from using commercial wood polishes. Over time, they tend to cause damage to your guitar.
Cleaning the Fretboard
You can run a damp cloth over the fretboard while you are cleaning the rest of your guitar, and it will remove the top layer of dirt, but to really clean the finger grease that has built up on the fretboard, you will need some type of cleaning agent. Lemon oil works well as a fretboard cleaning agent as does a formula of 1/3 vinegar to 2/3 water. You can use a cloth to apply this cleaning agent, but to really clean the fretboard well, you may opt to use steel wool. If so, ensure that it is the finest grade available because harsher grade steel wool can damage your fretboard.
Cleaning the Electronics and Other Places
The metal sections of your guitar can also be cleaned with a damp cloth. If this does not yield the required results, you may want to use a mild metal cleaner or finish. A few drops of these products can go a long way toward cleaning your guitar's metal parts; just don't use too much. Cotton swabs can be used to clean the more hard-to-reach places on your guitar, like certain places on the bridge and in the pickup cavities.