Lubricating your string slots in the nut is very helpful. You can simply take a graphite pencil and color the slots. This will allow smoother tuning and help eliminate the “creaking” sound you get when you tune your guitar, if the slots are slotted correctly.
Once every season I usually clean the fingerboard and polish the frets. I do this on every guitar I repair because I believe that it can not be done often enough. Doing this will prevent finger grime build up and prevent your frets from getting tarnished. There are two schools of thought here. One is to use an oil that dries hard and the other is to use an oil that does not dry. A maker may use oil that dries hard, like boiled linseed oil, in order to get a nice polished fingerboard on a new guitar. To continue using a hard drying oil, however, is the wrong approach. I like to use mineral oil, a non-drying oil, to clean and condition the fingerboard. It soaks in to the wood and acts like a solvent to bring dirt and moisture to the surface. Mineral oil will also prevent the fingerboard from absorbing excess moisture, creating a more stable neck.
If there is a lot of dirt built up between the frets, try a little elbow grease with a soft rag and mineral oil to get rid of it. Sometimes if the dirt is too old and set you need something a little stronger. The popular “Goo Gone” works very well in releasing hard to clean spots and grimy dirt. Put a little of the solvent on a rag before you begin. It is not a strong solvent but leaving any on the surface of a French polished surface is not a good idea. Another cleaning element I use for excess grime is extremely fine steel wool. The OOOO grade works fine but a company called Liberon makes a product that has an even higher grade and is much more consistent. Steel wool is great for quickly polishing frets and even polishing the fingerboard. To avoid scratches, make sure you are using it in small strokes between the frets going longitudinally to the neck, in the same grain direction as the wood. I usually clean the board using mineral oil, then use the steel wool to polish the frets and the board, and then follow it with another light rubbing of oil. Let it sit for fifteen minutes and rub it down with a clean cloth. Doing this every few months will make cleaning your finger board much easier and overtime you will notice that you don’t need to use much oil.
Cleaning the surface of your guitar is different than polishing. Cleaning is a routine task that anyone can perform and should be done after long playing sessions to remove sweat and finger oils. Polishing on the other hand, is a very fine abrasion process or a re-dissolving process that physically changes the finished surface. If you have acidic sweat and oils in your body, cleaning should be done after each session, including wiping down the strings. I have seen beautiful finishes, even hard oil varnishes, eaten away from acidic hands and body parts. A great polish or cleaning solution to use is called Preservation Polish which works very well on all finishes. It is also fairly inexpensive for the amount that comes in the bottle and contains no silicone. Silicone traps dirt and is an enemy for a repairman.
Humidity is a topic that can be discussed forever. In short, your guitar should be kept at a constant level around 40-55%. Have you ever noticed that old windows or doors work really well in the winter months and then suddenly wedge themselves stuck in the hot summer months? Well, guitars have a similar movement due to moisture changes. Guitars sound the best in the atmosphere that reflects the environment in which they were constructed. I tell people that it is not bad if their guitar goes below or above this percentage range, but it is horrible if it does so with a rapid change. In places like Chicago, Illinois it can be raining one day and then snowing the next. To battle this, one must control the environment in which their guitar is kept. Leaving it out of a case in a room is no different than leaving it in its case: it is just more work to keep a room controlled than a case.
I like to see cases humidified rather than seeing guitars humidified. By putting a wet sponge or some humidifying device inside the guitar you are centering the highest amount of moisture inside the box of the guitar. This can cause a rapid change in the top of the instrument and should only be used if that is needed. The neck needs to be controlled just as much as the sound box and humidifying the case insures that there is a more even distribution of moisture. A guitar that is humidified too much exhibits a loss of clarity and response in tone. If you don’t already have a digital hygrometer (a moisture meter), it is a great investment.
Each one of these topics can be talked at great lengths with many different directions and opinions. These ideas have led me to better results not just within the world of classical guitars but also steel string and electric guitars, violins and cellos. The one uniting principle behind all of this is observation. The more you observe, the more you are open to obtaining knowledge and building a better relationship with your guitar.