As you may know, Android handsets currently available tend to have limited amount of RAM. With multitasking built into Android, the RAM can get filled up very quickly. When the OS detects that the memory is close to full, it starts automatically killing off background applications to free up more RAM for the currently active app.

That is why often when switching from the Browser and coming back you will see the page refresh. It's actually reloading the Browser application because it's been terminated. Sometimes the home app (Launcher.apk) gets killed off in this process, and that is why you see it start up all over again when you exit a particularly intensive application.

There is very little you can do to help with this situation on a non-rooted phone:

1. The main advice is to not install too many applications. Since non-rooted phones are limited to using internal memory for applications, the more apps you install - the less memory will be available to the OS for multitasking. This in turn causes background applications to get terminated more frequently. By having fewer apps installed you minimize the need to terminate background apps often.
2. Clear the caches of Browser, Google Maps, YouTube, and other frequently used apps often. Since caches are stored on the internal memory, the same idea from the above point applies.

Rooted phones have a lot more options:

1. You can enable Apps-to-SD so that your applications get installed to the SD card's partition and therefore don't use up as much space on the phone's internal memory.
2. You can enable Swap File or Swap partition to extend the available memory by having the phone keep faster access to the background apps by temporarily moving their memory footprint to the SD card.
3. You can enable the Compcache module, which basically compresses part of the phone's memory, thus giving you more "virtual" memory in return for slightly heavier CPU usage.
4. Overclock the CPU. There are apps/widgets available to make the CPU run faster than the stock speed (actually at fully rated speed of the G1/MT3g's CPU, 528 Mhz). It will not help with memory management, but it will at least make apps (re-)load faster.
5. Finally, there is a hack available here that adds extra 10% to the phone's memory (~10 MB) by reassigning the memory that's normally dedicated to the 3D subsystem. Unless you plan to run some heavy 3D games on your G1/MT3G, this hack has no drawbacks.