So I guess one thing I'll start off with is that I've found that search engine friendly URL's make a big difference in your ranking, traffic, revenue, etc.
If you're using .php I'd recommend a modrewrite so that all of your URL's end in .html
Example:
http://www.yourdomain.com/category/d...hp?itemid=1234
should be:
http://www.yourdomain.com/1234/Name-Of-Item.html
I've had very good success with this method on most of my sites - and I'd recommend everyone else to do the same. The other sites I had that had the first link structure really dropped off and didn't draw much traffic, whereas the newer link structure has ranked better and had consistant traffic. Please keep in mind that your pages should be set up so that your titles, descriptions, keywords, etc. all be different and related to what each page/product, are about.
A very good book for those of you interested (at least the one that I bought that I found helpful) is this one:
(The above is not an affiliate link - hopefully you find it useful)
Hope some of you find this info useful. And I encourage others to post their tips and experiences here as well.
Ian