Noone has mentioned building the SPF strings :) I found this a while back:

http://www.anti-spamtools.org/SenderIDEmailPolicyTool/Default.aspx

There is wizard on one of the one links posted by Craig. http://spf.pobox.com/ , http://www.openspf.net/ (some thing) The wizard is on the top left, where it says to enter your domain. - Drew