1. Use the right markers. This is probably the single most important thing you can do to keep your board in great shape. Make sure you use markers that are specifically meant for dry-erase boards. It should say right on the pen whether it's a dry-erase marker or not. (Your instruction manual may also have suggestions on which markers you should use.) Make sure you don't use Sharpies, highlighters, or Mr. Sketch markers. If you do, there's a good chance you'll never be able to get your writing off the surface, leaving you with a ruined whiteboard.
2. Erase your board regularly. After using the right markers, using the right eraser regularly is very important. Be sure to use erasers that were specifically designed for whiteboard use. They're non-abrasive and remove dry-erase ink much better than any rag or towel ever could. Also, make sure you erase your board every day. This will prevent it from becoming stained. And if you need to erase your writing just after it appears on the board, wait about 10 seconds before doing so. This gives the ink time to dry and it erases better when it's not wet.
3. Use a commercial cleaner. It's pretty easy to clean your board, especially if you use a commercial cleaner. There are several on the market and the particularly good ones are made by Quartet, a manufacturer that also happens to make very high-quality whiteboards. Their cleaners are available in a 2-ounce spray bottle as well as wipes you can use to literally wipe your board clean. There's even a cleaner that will remove permanent ink if someone accidentally uses the right kind of marker. Whatever you do, don't give in to the temptation to use household items such as nail polish, baby oil, WD-40, or other liquids to clean your board. Not only can these things possibly damage your board, they could be hazardous to your health as well.
4. Clean it often. Generally speaking, the more you use your whiteboard, the more you need to clean it. If you use it every day, you should clean it a few times a week. If you lose it less, you'll be able to get away with cleaning it less often, maybe one or two times per month.

Maintaining your whiteboard is actually pretty easy, so long as you have the right supplies on hand. They key thing is to use products that were specifically designed for use with whiteboards and to use them on a regular basis. If you do that, your whiteboard will be able to be used for a long time and it will also look great!a