Here's a quick tip for high traffic eContact Centers:

The first question on every Web-to-Case form should always be "Have you already searched the knowledge base? Yes/No" with a link to the KB immediately following.

While I haven't done a page abandonment analysis on this to prove it, I estimate it reduces the total number of submitted cases by 10%-40%.

Tip #2 (even better):
Prior to submitting the web-to-case form, have the customer enter a problem description then search the KB on the customers behalf; showing the top 10 most relevant articles. Then ask the customer if the articles did not address their problem before allowing them to submit.