This article series is kind of a Bible to me. It's not going to solve all of your problems, for sure, depending on yer stack, and setting yourself up for a forkbomb isn't the wisest in all situations, but it's got a lot of good advice & is pretty good about providing you a "Okay, tweak these parameters and then try to break it" baseline.
http://www.metabrew.com/article/a-million-user-comet-applica...
Part 3 is my favorite. http://www.metabrew.com/article/a-million-user-comet-applica...