Redirect of non existing pages

old cached pages that no longer exist - what to do?

[5x54u]

Over a year ago I re-designed a site and converted from asp to php, I redirected base urls (non dynamic pages)with 301's to new pages/dirs.

I am seeing in several Yahoo SERPS the old cached version of some of the old dynamic URLS that are showing up in competitive SERPS.

What is the best practice for IIS to handle these dynamic requests that no longer exist?

Thank you in advance.