When I try to set my preferred domain in Google webmaster tools I receive a message that states "Part of the process of setting a preferred domain is to verify that you own http://mysite.com/. Please verify http://mysite.com/. My site is of course already verified but I returned to the webmaster home page and re-verified my site. Went back to set my preferred domain and received the same message. Can someone tell me what I am doing wrong? Thanks.
Goto Dashboard of Google Webmaster Tools and add site twice, with both URLs, with-www and without-www, and verify for both, because google take as different site.
They just told you how to fix it? You have to separately verify that you own the www as well. Create an account for www and verify you own it.
do you have a 301 redirect already from non-www to www ? if so you will have to remove the 301 then verify and set your "preferred domain in Google webmaster tools" than after that you can re-add your 301 redirect
you can use a header checker tool and see the response code you get. let us know the domain so we can check if there is a redirect.