OK, there are no secrets; I categorise the simple way... manually! I realise that the whole point of blog software is to automate simple tasks, but I am used to manually creating web sites and so do not shirk from a little heavy lifting. In this case it's only one extra step for each article.
To set up the system I first decided on categories, which may in fact have been the most difficult part of the process. I created entries for each category, and back-dated the creation times so the articles became lost in the bowels of the archives. Then I put links to them in my sidebar, so the category pages are easy to access. So far, so good.
Every time I add an article I must now also edit the appropriate category page and add a link to it. This is not too much work, IMO. Sometimes I wait and do several at once, since categorising brand new articles, those people can easily read on the opening page, is not a high priority.
Blogger should definitely have an automated way of dealing with this. It's trivial to implement so I don't understand why it hasn't been done.
However, the manual method has certain advantages. There is full control over how you link to the categories, full control over their contents and formatting, and it's easy to put an article in more than one category if you wish.
I am not keen of categorising using hacks that depend on outside resources, though if you want to check that out, some techniques are available over on FreshBlog.
Another technique that makes this method easy is explained in the next article, Quick Article Editing. (This is getting nicely entangled.)