Wash your hands.
Surprisingly, the number one tip for keeping your guitar clean is to wash your hands before you play the instrument. Not only does this remove any dirt and grime on your hands, it will also reduce the natural skin oils before you play. All of this helps keep the guitar clean.
Wipe down the guitar
Each time, after you play your guitar, you should wipe it down with a soft cloth. Dedicate this cloth to wiping down your guitar and don't use it for anything else. Pay particular attention to the strings: you will find that the strings last much longer if you wipe them down at the end of each playing session. There are special 'guitar wipe down' cloth products that are specially treated to make this an easy task.
Get a paint brush for dust
Take a trip to your local hardware store and purchase a soft-bristle paint brush that's about 2-inches wide. With this brush, you can whisk away dust that accumulates under the strings at the headstock, as well as under the strings near the bridge. Please use a new paint brush for this task; don't try one that's been used for touching-up the baseboards around the house!
If your guitar already has smudges and grime from some period of practice and play, use any quality brand of Guitar Polish, such as the products from Martin, GHS, and Planet Waves. Dedicated guitar-polish products are specially formulated to clean and preserve the finish of your instrument and can restore the "like new" sparkle of your guitar. Do not use any products that contain silicone (such as Armor-All).
Car wax and furniture polish
Simply stated: don't use this stuff on your guitar. These types of products can cause a wax buildup that is difficult to remove, and may cause long-term damage to the finish of your instrument.