Main page: http://help.yahoo.com/l/us/yahoo/mail/postmaster/
Feedback loop
Get a notification when users flag your e-mail as spam.
Start here: http://feedbackloop.yahoo.net
Login with a Yahoo account, which can also be via a Google account since it supports OpenID (but to use OpenID, sign-up from the front page of Yahoo.com...)
You then need to fill in a form to subscribe to the service, and it will request to confirm your e-mail address.
?
DKIM
Yahoo is very insistant on using DKIM.
Howtos: