The best guides I can offer are the documentation sites and the configuration files, which are generally self-documented fairly well. Look through the settings and see if they make sense.
As an example, I think the default Apache setting for minimum spare servers is something like 5. So you've always got a minimum of 5 web server processes sitting there doing nothing but occupying memory at any given time. If you have a busy web site, that might make sense. But if you're only getting four visitors an hour, it probably doesn't, because it only takes a few seconds to spawn another server. I set my minimum spare servers to 2.
A tool to help you is the Apache benchmarking tool (http://httpd.apache.org/docs/2.2/programs/ab.html) which will give you a pretty good idea of how your server will respond to your configuration changes. In addition to the performance data that you get from ab, you can run 'top' in a terminal window while you run ab and see the load on the processor and watch how much memory gets swapped, and see if you're happy with that.
I am far from any kind of a learned expert on this stuff. Everything I know, I learned from reading the documentation, reading the config files, an occasional Google search (stackoverflow.com has a lot of answers on a lot of topics), and a lot of experimentation.