Aside from the no-cache pragma, which I find works for 98%+ of folks out there, I find the simplest and easiest way to create a unique string is just a call to time(); e.g.,

$url = '/cgi-bin/script.pl?time=' . time();

Using both of these things in combination results in 99.9% effectiveness in preventing caching. (Estimates based on a scientific survey created by top scientists which was just recently pulled out of my *.)