I have seen a plenty of thread here on http://presta-tr.com/forum that talks about perfect structure for urls. I have done a research about it from Moz blog, and google official blogs to find out the tips for structuring urls. You will see the reference links at the end.

1. URLs shall be readable
2. Eliminate IDs and numbers
3. Canonicalize URLs directing to a common content
4. Cover your keywords
5. Harmonize the URL with the page title
6. Exclude stop words from the URL
7. Limit the number of folders
8. Use word separators
9. Set moderate redirections
10. Consolidate the www and non-www URLs

Reference Links:
• Reference Links:
• https://moz.com/blog/15-seo-best-pra...ructuring-urls
• https://support.google.com/webmaster...er/76329?hl=en
• http://www.fmemodules.com/en/blog/55...er-infographic