Get a guitar stand. This is actually part of cleaning. If the guitar is laying on the floor or somewhere else it can get damaged or collect dust.
Step2
Keep the guitar in a non-humid room (around 50% humidity) and room temperature—the attic or the basement isn’t your best bet. An overly dry environment isn’t good either, you may need a humidifier. A guitar is made of wood, which can swell or crack depending on the environment.
Step3
Clean the guitar with a damp micro-fiber cloth—not your standard dishrag, but a soft cloth. Guitar stores sell cloths specifically for guitars.
Step4
After cleaning the guitar, use a guitar polish. This isn’t recommended all the time as some guitar polishes can strip the finish over time. Absolutely stay away from 409 and other household cleaners. Mild furniture polish can work, but guitar polish is recommended.
Tips & Warnings
Keep the guitar in its case, if not on a guitar stand. Some people just like to have the guitar on display.
When you change strings, clean the fretboard—an argument for changing strings frequently so there isn’t too much buildup from oil and dirt on the fingers. Don’t scrub the fretboard, even if it’s dirty—this can damage the fretboard’s natural finish.
Check your guitar’s manufacturing guidelines—each guitar, acoustic or electric, may have different requirements or warnings about cleaning.