The process is closer to trial and error than to formal experiment. However, I have a ton of objective evidence in the server logs and a nifty piece of software by WebTrends that lets me slice and dice it every which way. I also have the results of the user, site, and page analysis forms in this section. They tell me more than I can handle. While I'm saving most of this primary research, I'm still discovering how to best use it.
Suggestions are always welcome.