If you right-click Computer and then click Manage. Then click Disk Management you can assign a drive letter to your external drive.
Choose a letter high up the alphabet, and that drive *should* get the same drive letter each time.
I have just tested the theory with a few different USB drives. I assigned one as T: and unplugged it and plugged it back in. Was assigned T: again. I also tried it in a new USB port, again, assigned T:. Even with different combinations of drives plugged in, and changing the order I plugged them in, the same drive was always assigned T:. This was on Windows 7. This behaviour may not be the same on XP or Vista.
I am not saying it is fool proof, but my quick experiment showed promise