My reference for url structure...

I’ve decided to make up a short all-in-one guide to summarize what we know about SEO for URLs. And if you have something to add, please do. So he we go:

• Length of the URL: no more than 3-5 words in your URL. According to Google’s Matt Cutts if there are more than 5 words…

[Google] algorithms typically will just weight those words less and just not give you as much credit.”

Here is one more evidence in favor of short URL: recent research shows that short URLs within Google SERPs get clicked twice as often as long ones. So by sticking to short URLs you get both better rankings and better clickthrough.
Short URLs will also help in direct type ins of URLs (if anyone still uses that instead of Google).

• Dashes are better than underscores. Although Google has no individual preferences (meaning you won’t be penalizes for either of the versions), dashes are more preferable as Google “sees” each hyphened word as an individual one:

So if you have a url like word1_word2, Google will only return that page if the user searches for word1_word2 (which almost never happens). If you have a url like word1-word2, that page can be returned for the searches word1, word2, and even “word1 word2?.

• Unlike a domain name, URL is case sensitive – meaning that if by any reason (your choice or CMS) you stick to a an upper-case version, remember that this can cause a few issues: people are most likely to link to the standard lower case one and you might both lose link juice and suffer from duplicate content issues.

• Moving to static URL structure: my (and actually not only my) favorite tactic is to use 301 redirect only for most powerful (in terms of linking and traffic) pages and leave all others to be handled via 404.

• If you hesitate if your URLs may be perceived as spammy, check out SEOMOZ URL Spam Detection Tool that will estimate:
  • spam words;
  • hyphens;
  • subdomain depth;
  • domain length;
  • digits.
• Mind your file extensions (i.e. don’t end your URLs with .exe) as they might prevent your pages from crawling.