I had the pleasure of meeting Elizabeth Leddy for the first time at the Plone Symposium East conference in Penn State. I also got to sit through her talk about "Unloading Plone: Tools, Tips and Techniques for Approaching Scalability in Plone Systems" and learned quite a few things. You can find the slides and recording of the presentation on the Weblion site.
Elizabeth told us about some simple things you can do to monitor your site and pointed us to some tools to use, Munin being her favorite. The first easy step is to receive an email each time there is an error in your Plone site or have a Google Analytics account setup to track pages that error out.
She also spoke about caching and how to make your Plone site faster with it. She recommended reading "Web Caching" book, by Duane Wessels, to understand better how caching works.
Finally, she gave two pieces of advice that I thought were worth repeating: "Restarting zope is not a sin" and "I will not do anything manually that could be done automatically". You can find more details on her Scaling Plone site.