I hear you asking, "But which caching plugin are you currently using?". I'm now using a plugin called Lite Cache. Since this is the plugin I am currently most familiar with, I'll go into a little more detail on how to set it up properly.
First, download and network-activate the plugin, which you should know how to do by this point. I'm going to enable Lite Cache for Tyler's Blog, so from your main dashboard
(/wp-admin/), mouse over My Sites in the top-left of the screen, then mouse over the blog you want to enable Lite Cache on, and then click on the Dashboard submenu item. Since, we network-activated this plugin, we don't have to activate it on the individual blog.
So, mouse over the Tools menu item in the left-hand menu, and then ...