How do you fix “not found errors” for your website ?

I see a number of "not found errors" for my website inside Google Webmaster's tools.
I have a niche classifieds site and these errors are basically for the page listings that expired .
I setup a notification saying The Ad has expired for each of these pages but i believe it's still harming my rankings on google.
What's the proper way to fix them?

See original post by dpuser09