SEO Tips when planning out your URL structure:

* Avoid burying your content deep within your site. Use fewer folders, try to keep your important site content less than three or folders deep. If you don't treat your content as important why would you expect the search engines to?

* Describe Your Content - to promote the theme of your site, where possible try to ensure that your URLs accurately reflect the page content.

* Shorter URLs are better than longer ones for usability as they are more easily remembered.

* Avoid use of numbers as opposed to using descriptive keywords. The exception may be incorporating the date into URL. Of course with blogs and archived data, many URLs tend to contain a number sequence reflective of a defined time period.

* You can use underscores ("_") or hyphens ("-") but hyphens tend to look better and are preferred by many (some say the search engines even prefer the use of a hyphen over an underscore.

* Sub-domains can be effective if you have the supporting content and "theme" to back them up.

* Static URLs tend to be shorter and more effective than dynamic URLs. Search engines have been pretty forward in saying that they prefer static URLs.

URL structure is something that may take a lot of planning when launching a new site or contemplating a site re-design. Consider taxonomy and hierarchal structure, classification, and grouping. Shari Thurlow had a great piece on URL structure a while back. Proper URL structure is one of those items that can have a great impact on your site rankings in the long run.