A lot of people who are into search engine optimization vary on this. Personally I use /%postname%/ and set it up for my own clients. Why?
It is easy and contains the keywords necessary for the optimal search. I am not known as an expert, but I have built quite a few sites that have improved well through ranking. I believe this has been the best. I know there are people that like %postname%.html and /%category%/%postname%/.
Why is it not as great to put the date or category in the URL structure?
Well, you are telling the search engine that the day and the category are more important than the article itself. For those who write tutorials and helpful topics, it is not the date or category that people are looking for in the search engine.
The great thing about WordPress is that is is easy to choose a custom structure and while posting, you can have a page slug that sums up the article, while the actual post name is longer. A lot of times this is to focus on keywords.
By the way, I really think that if you have not, you should check out Matt Cutt’s article Straight from Google: What You Need to Know. If you noticed, Matt Cutts uses a post name structure, but he also utilizes custom slugs where his URL path contains keywords the sums up his post, and his title is different. The article contains a slide show and video too. Another person I recommend that has a lot of experience is Joost de Valk.
As a note, with WordPress 3.3, this ability to select the Post Name structure had been added, so you no longer have to manually type a custom structure.
What permalink structure do you use for your website and why? Any SEO tips for WordPress users?