The most basic and often overlooked guitar maintenance is cleaning. Your guitar should be cleaned regularly, or whenever it gets dirty. Keeping your guitar clean will lengthen its life, improve its resale value, and make you look way cooler than your drummer sitting behind his dingy drum set.
Guitar shops are happy to sell special polishing cloths, but a basic 100% cotton rag (or worn out tee shirt) will work just fine. That 4-set gig you played on Saturday night in that sweltering club was as tough on your guitar as it was on you. Sweat is an oily and salty concoction that will cause your guitar to age prematurely. Try wiping with a dry rag first. If you need a little more "oomph", spit works nicely and is surprisingly affordable. If spit isn't enough, or you're suffering from extreme cottonmouth, you can dampen the rag slightly, but be stingy with the wet stuff!
Your guitar's strings lead a hard life. The natural oil, sweat and dead skin from your fingertips coat the strings every time you play. This gunk corrodes the string material, deadens their tone, and shortens their usable playing time. Left unabated it can actually seep into the pores of the fretboard. The best way to combat this grimy menace is to wipe down the strings after every playing session. Just pinch each string between your thumb and index finger, with the cloth in between, and run your hand up and down the string length. This dries the string all the way around its circumference and scrapes off any grunge.
I know, I know. It's 1:00 AM and you just finished playing another 4-hour gig. Your back hurts, your head is pounding and your ears are ringing. Here's a little tough love. Suck it up and tidy up your guitar anyway. For a professional, a guitar is a tool and will serve longer and better with a little attention. There's an old saying: "Rode hard and put away wet". It used to be about taking care of your horse.