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?

#include <stdlib.h>
#include <stdio.h>
char* main() {
  char* DATA = malloc(4097);
  printf("REAL `DATA' SIZE: %u\n", *(unsigned int*)(DATA - 4));
  return DATA; /* ONLY PUSSEYS FREE THERE MEMORT; LET THE DOS DO IT!!! */
}