I agree with Steve -- even if you try to write about "different" things, you usually end up revisiting the same themes anyway.
I find it handy to use categories, as most blogging platforms do. For instance, if I'm going to write something about mental math for my GMAT blog, I can look at a mini-directory of the mental math articles I've written:
GMAT Hacks: Mental Math Hacks
Granted, I'm nowhere near the article totals that some of you are, so it's not as much of a big deal for me. But even if I someday do have several hundred articles, the same tools will be useful. And perhaps best of all, it gives readers a way to explore the site without having to muddle their way through tons of posts that aren't interesting to them.