Try and make sure the sites you're visiting are optimized for mobile, open fewer tabs at a time, and other ways of not using Safari as heavily. Not exactly very suitable answers until iOS 4.1 is eventually released for the iPad. Alternative browsers such as iCab have an offline bookmark option that will save the entire page to disk so you can view it without needing to refresh until you want to.
Safari on iOS has a very limited size on it's cache and will refresh a page when you come back to it if the cache is full. Part of this is because of the limited RAM available on iOS devices along with the fact that Apple doesn't write any cached items to the flash memory and only keeps it in the memory. (The iPad and iPhone 3G S only have ~256 MB of RAM, earlier devices 128 MB and the iPhone 4 512 MB)
Yahoo has run a series of tests to determine just how large the cache is. If the HTML of a page exceeds 25.6KB it will not be cached (and so refreshed every time you visit), external components can be larger (Up to 4MB) but the max size of the HTML page cache is only ~280KB on an iPad (paltry really). iOS 4 is set to increase that page cache size up to as much RAM as is available.