Use a clear and consistent “From” address. Put some copy in your e-mail asking the recipient to add your “From” address to their address book to ensure delivery.
Here are a couple of websites that I have run across that give specific instructions on how to add the companies “From” address to the recipient’s address book for some of the top email applications.
ReturnPath also published a whitepaper on getting your recipients to add your “From” address called, “The Secret Role of the Email Address Book… and what it means for you email delivery”.
eINFO – a collection of articles and studies about email marketing