Avoid Keyword Stuffing in the URL
If you use keywords more than once in the URL, search engines may tag your page for keyword stuffing (a form of search engine spam), which can hurt your rankings in the search engines, as well as appear spam-like to potential visitors. Keyword stuffing also makes your URL longer than it needs to be, which can have a negative effect on users and search engines alike.
Recommendation: Rewrite your URL to use your targeted keywords no more than once. See "Keyword Stuffing" on this page: http://moz.com/blog/should-i-change-my-urls-for-seo
URL Uses Only Standard Characters
If you only use characters that are common in URLs, it makes it easier for more users to access and interpret your URL. Not all users have keyboards that can easily enter less common characters, or browsers that support the display of these characters, and some special characters can look spammy. Using only standard characters can also avoid potential problems with your search engine ranking.
Recommendation: Make sure your URL contains only these characters: letters, numbers, slash (/), comma (,), plus (+), exclamation point (!), period (.), and dash (-). Parameters using ampersand (&) and pound (#) are okay too.
Use Keywords in Your URL
Using your targeted keywords in the URL string adds relevancy to your page for search engine rankings, assists potential visitors identify the topic of your page from the URL, and provides SEO value when used as the anchor text of referring links.
Recommendation: Use your targeted keywords in the URL string of the page. If you are targeting a multi-word phrase, use hyphens to separate individual words. Hyphens allow the search engines to read the URL as separate words. See http://moz.com/blog/11-best-practices-for-urls
Use Static URLs
Using a static URL can improve your performance in search engine rankings. Moz's correlation research shows that URLs with dynamic parameters have dramatically worse performance in the rankings. Using dynamic parameters does not necessarily cause worse rankings, but there does appear to be a correlation, and they generally do lead to lower click-through rates. Dynamic URLs are also a common source of duplicate content.
Recommendation: Use mod rewrite or ISAPI rewrite to change your URL to be static (removing all instances of ?, =, etc.). See http://moz.com/blog/url-rewrites-and...es-it-all-work
Minimize URL Length
Search engines often truncate the URL display at 75 characters and appear to pass less keyword value in longer URLs. We also recommend using fewer than three subfolders in your URL to make it easier for search engines to parse.
Recommendation: Rewrite your URL to be as short as possible, preferably shorter than 75 characters and three subfolders, while still being descriptive and using keywords appropriately. See http://moz.com/blog/11-best-practices-for-urls
Lots of reading at additional links.