"I've been writing C for quite some time, but I never followed good conventions I'm afraid, and I never payed much attention to the optimization tricks of the higher C programmers. Sure, I use const when I can, I use the pointer methods for manual string copying, I even use register for all the good that does with modern compilers, but now, I'm trying to write a C-string handling library for personal use, but I need speed, and I really don't want to use inline ASM. So, I am wondering, what would other Soylenters do to write efficient, pure, standards-compliant C?"
This^N. Get something that works first, then figure out how to make it better.
But having said that, your first few passes through the profiler may be quite enlightening as to where your program is spending its time. If I were profiling this string library, I'd start by just performing each distinct operation of the code a few thousand to few million times each, including creation and deleting of strings. Also, if your code is optimized for certain types of strings (eg, long string operations - see forsythe's comment [soylentnews.org]), then try it on stuff that it isn't optimized for (like short strings).
Stress testing (where you keep performing zillions of operations until something breaks) would be useful too. This is a more likely way to catch memory leaks and other subtle problems that slowly build up over time.