In the Store: With so many different geometry styles, the only way to properly size a mountain bike these days is to ride before you buy. Most shops have demo models. » Don't automatically buy the most travel you can afford. Consider where and how you ride, and get the best suspension (full or hardtail, more or less travel) for the conditions.
In the Field: Get tire pressure right. Generally speaking, lower air pressure equals better traction. If you start getting pinch flats, though, increase the pressure in small increments. » Lube early and often: Doing so will keep your bike out of the shop longer. » Shock systems continue to get more and more idiot-proof. But that doesn't mean you can't still be an idiot: Make sure you know how to adjust them before you head out.
At Home: Clean your bike after every ride. And don't let minor derailleur issues become major ones. If you can't fix it, head to your more mechanically inclined buddy's houseor the shop.