I use to have a problem with this iPad closing apps. I cleared cookies, I closed apps in memory, and when it got too bad, I turned off my iPad. These were all good workarounds. However, after updating my iOS to 5.1, many of these problems have gone away. In the two days of using this iPad since the update, I haven't experienced Safari closing-->I haven't cleared my cookies yet. And all heavy memory apps do not advise me to close all apps and reboot my iPad. I still clear out apps in memory.
Your experience might be different, but I highly recommend upgrading to the new iOS5.1.