Google Inside

SEOmoz has some great tips for URL structures and the best practices.

1. Describe Your Content
2. Keep it Short
3. Static is the Way & the Light
4. Descriptives are Better than Numbers
5. Keywords Never Hurt
6. Subdomains Aren’t the Answer
7. Fewer Folders
8. Hyphens Separate Best
9. Stick with Conventions
10. Don’t be Case Sensitive
11. Don’t Append Extraneous Data

I agree with most of these, but I’ve had pretty good luck with how I structure my URLs.