The iPad's operating system is designed to automatically shut down apps which are consuming high memory resources, or when memory resources run low.
First, check which version of operating system you have (Setting > General > About). If it is version 4.2 or later, then you can double-tap on the home button. This brings up the multitasking bar at the bottom of the screen - this shows which apps you have currently in multitasking state. Touch and hold one of the icons until they start to wiggle, then shut down the apps which you don't need. If you have an older version of operating system, it's worth considering upgrading to a more current version.
I also use the app XSysInfo which reports on memory usage, and it also offers two levels of memory cleansing (normal and deep) - it's like doing a reset to clean out your memory except without having to actually reset. It will work on iOS 3.0 or better, so chances are you should be able to use it as well.