C programming tricks

While programming optimization that makes ones' code completely unreadable is often a bad thing, there are places for it, like that one inner loop of your code that takes up 98% of the program's running time.

What are you tricks for improving performance in C, other than the obvious inline assembly or the like?

are we talking compilation optimization or code execution? because i don't really care how fast the compile is, in my case i've so far only written applications that compile in a few seconds or less

so what tips do you have for optimizing executable code when you compile with gcc and link with ld to the bsd libc?