I'll say to you what I tell everyone (including my clients)...
Use less categories and more tags.
Put it this way, let's say you have a food blog where you post recipes. Assuming you post many other things aside from recipes, let's say you're going to post a dinner recipe, made with peppers and onions, and it's Italian in "flair".
I'd probably have a category for recipes, and a subcat for "dinner recipes", and categorize it there. And ONLY there.
Then I'd tag it with "green pepper, onion, Italian, dinner, pasta, etc."
The reason being that every time you use multiple categories for a post, you're essentially creating a new URL for that post if you're using "pretty permalinks" that feature the category in the URL (which you should be doing, by the way, if you're not already).
Let's say you were to categorize the post the "wrong" way, and put it in "Dinner" "Italian" and "Recipes" using the permalink structure that includes your category in the URL... You'd wind up with three URLs that would link to the same post...
...yoursite.com/dinner/post-title/
...yoursite.com/italian/post-title/
...yoursite.com/recipes/post-title/
All three of those would resolve to the very same post.
ALSO - multiple categories make for longer archives that aren't needed. The archive page for each of those categories would include the same post, so if someone really only wanted to find food using green peppers, and they clicked on your Italian category, they'd be hit with a ton of things that didn't use green peppers. Whereas if they clicked on the green pepper tag, they'd find ONLY what's including green pepper.
Does all of that make sense??