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?

>>40
low bits of values returned by rand() aren't very random so if you're going to use rand() instead you shouldn't use just the lowest bit.
& is uglier than %, and just as slow.
if you really want speed, rand()/(INT_MAX/2) would be faster and would help with the problem of the low bits not being very random.
improved version:
for(;*s=(rand()/(INT_MAX/2)?toupper:tolower)(*s++););