Just tap on the Home button to close the app. Most apps will be frozen in RAM, not running, but ready to start where they left off. When you want to go back to it double tap the Home button for the multitask bar and select the app.
As long as you don't start a lot of apps in between, or start an app that requires a lot of RAM (iPad 1) or humongous amounts of RAM (iPad 2), this works fine. Also check out Multitask Gestures for quicker ways to move in between recent apps.