good idea, but don't some controllers have an unprotect mode?
yes, they do, it is called 'erase' anything else would be outright silly.

2) Include some simple things in your code that make it harder to simply blot out things like a copyright string. One example is to place a CRC-32 value into the image of the FLASH and have that be checked at bootup across the whole flash space image.

Thats good as well, but it requires more time from the programmer, and the program speed might be slower because of the time it takes to decode the special data (like crc-32 values).
who gives a hoot, it is "at bootup"

4) Write the code in C language.

I dont think that this will help him resolve his problem.
I would, I could recreate source from assembler code in about 1/27 of the time it would take to recereate source from C. If you want to make it REALLY tough use higjhly optimized C

Erik