Know that when you increment a pointer, the new address depends upon the size of the data pointed to by that pointer... (IE, what's the difference between a char* being incremented and an unsigned long*)...
Knowing exactly what a segmentation fault really is first of all, and also how to deal with them.
Knowing how to use GDB is great. Knowing how to use valgrind is great.
Develop a C programming style... For example, I tend to write fairly object oriented code when I write large C programs (usually, all the functions in a particular .C file accept some (1) particular struct* and operate on it... I tend to have foo* foo_create() and foo_destroy(foo*) ctor's and dtors...)...
