(Disclaimer #1: if you're not a blogger, or are one but don't use Blogger, then this posting redefines the Platonic essence of "boring." Enjoy!)
(Disclaimer #2: I'm not qualified to judge or write about one of these two methods for creating categories, but that's never stopped me before.)
Filing posts in various categories that one writes about often - science, politics, lack of dates, evading parole officers - can be a useful trick for finding old posts. Two of my favorite blogs, Deltoid and Brad DeLong, do this quite a bit. Blogger, who provides my host software Blogspot.com, doesn't provide this capability directly. Since this website is free for me I can't complain, but still.
So, two work-arounds. First, Smack describes a system where you set up a separate blog for each category, then aggregate the blogs together in an uber-blog using the RSS system that I've never bothered to figure out. Seems pretty good.
Second idea is what I'm doing - I've added Google searches of this website to the sidebar on the left, with a different Google search for different categories. I've started to add keywords to posts so they'll reinforce the Google search. Seems to work ok - the only problem is that site is archived a month at a time, so you don't immediately see the post you're searching for. That requires some other kind of fix, by someone better than me.