where to find classification and data structure advice

we are in the process of developing a classified ad website due to launch in about a month (yeah i know, this overcrowded market, but we plan to focus on a certain region which is currently under-served and plan to develop that specific market by combining various off-line and on-line methods)

and would need some good advice on how to structure the categories (of course this would come down to the market we are looking at but what i am seeking is a sort of general repository or advice) and also what kind of data that should go in each ad to be posted

i have looked around at 20+ websites all from the simple title and description settings to the viciously structured ones with lots of choices for the user to fill in

so anything that would come to mind or references to such advices would be great

Not that I wouldn't have some categorization, but you should focus more on the search engine. I was amazed one day, when I discovered that it was easier to find documents on the interenet with Google than with Find on my Windows PC until I installed Google Desktop. I don't get caught up in creating layer upon layer of folders.

Without knowing more about the categories it's difficult to make a recommendation. As Jeff said though, I would lean away from the traditional tree layout and use something like tags/labels to keep them organized. This should make it simpler to list the same ad in multiple categories, and provides an easy way for those viewing the ads to find what they're looking for. And again, make sure the search system is useful. I'm not a big fan of "advanced" searches that require multiple checkboxes and selections. I prefer a simple text input that still returns good results.