How can I get my Android Phone to use its SD card by default?
The sad but inevitable answer is that you can't. There is as of yet no way to tell Android to use the spacious SD card instead of the precious internal memory when installing new applications or saving messages.

One workaround that I've taken advantage of is installing an Android ROM and running it from the SD card using magldr. My SD card isn't of particularly fine quality so the OS runs quite sluggish, but it beats deleting all my SMS every day