I've never been able to keep a diary, but for some reason I began keeping a tobacco journal ever since I first started with the pipe, back in 1980. If you only smoke a handful of blends, there's probably little need. But I've always enjoyed exploring a wide range of blends and styles. And it's interesting to go back and see what I wrote about a particular blend when I was a fairly ignorant beginner (Example: My total description of Balkan Sobranie: "Rich, English-style non-aromatic; heady aroma"!) and compare it to how I'm tasting blends these days. I also love the fact that my memory of so many "lost" blends has been preserved in this journal.
Whenever I open a new-to-me blend, I note the blender/blend in my journal, along with the date and any relevant/fun facts about the tin (the age, where I picked it up, etc.). I also give each blend a number, so I always have a running total of how many blends I've tried. Then after I've had about 10 bowls, I just write a paragraph or two about what I think. Sometimes I mention the weather or if I'm traveling; it's nice to look back at a blend and remember that I first tried it while away on a trip to Denver, or whatever.
BTW, I do all of this in longhand, in a notebook I inherited from my father. (It's the only handwritten thing I do.) The only thing I don't like about this is seeing how badly my handwriting has deteriorated over the last 30 years!
This might be too anal-retentive for some of you, I know. But I thought it was worth mentioning, especially since I find myself frequently rereading it and referring to it when I'm recommending blends here.