Mobile Safari clears tabs from memory when it receives a low memory warning from the OS. For performance reasons, iOS doesn't do any paging to disk, so when something is moved out of memory, it's gone unless it's been saved. So the reason you have to reload pages is that other tasks on your iPhone (other apps or new browser tabs) are using memory, so Safari kicks the older tabs out of memory, and when you flip back to them, they need to be re-dowloaded.
This is system behaviour, and it can't be changed. However there are several apps that are designed to save pages for offline reading. I personally recommend Instapaper - very well designed, actively maintained by a great independent developer, and the service integrates with a lot of other apps, so you can save links from most RSS and Twitter clients, amongst other things.
The basic premise is that when you open a page you want to read later, you use a bookmarklet to save the page to Instapaper. Then open the app and it downloads a nicely formatted, text- and pictures-only version of your saved pages for reading at your convenience. It has lots of other features, including saving your position in an article and syncing it to any other Instapaper clients you use. Highly recommended.