Ask for inclusion at the start.
Don't wait until you begin sending messages. You want to be on that personal whitelist even before you send a confirmation or welcome e-mail. So, make your add-to-sender-list request right on the subscription form. However, you should still repeat your request in your confirmation and welcome e-mails as a reminder to those who overlooked this step or neglected to do it at opt-in.
Explain why whitelisting benefits the recipient.
Don't expect much action if you simply say, "Add our sending address to your address books or contacts lists." Show them the value of doing so, or what they'll miss if they don't do it.
Explain that adding your address to the personal whitelist will keep your messages coming. This works, if you make a compelling reason to the recipient that your messages have value. If you simply ask without showing any value, few will respond.
Add a quarterly whitelisting campaign.
This campaign should remind subscribers to add your address to their sender lists. It's also a great opportunity to test segmenting and targeting of e-mail messages if you are generally a broadcast sender.
Create segments for low responders -- those who seldom open or click, or recent openers or clickers who have stopped responding. Target those segments with messages that not only spell out the benefit of adding your address to their sender lists but also provide more explicit directions.
Track actions on these messages. Also, watch your delivery reports to see if delivery and inbox placement improve.
Include the request in every e-mail, but not necessarily at the top of the message.
In most cases, your preheader line - the first line of copy in the message - should show your call to action or sum up your e-mail contents, because it might be all the reader sees when viewing the message in a preview pane or with images off. Unless, of course, you're sending your quarterly sender-list message.
However, it definitely belongs in the footer information of every e-mail message, where you include other standing information, such as postal address, contact addresses and phone numbers, etc.
Every e-mail should have add-to-sender-list language in the footer.
Again, don't just ask to be added. Highlight the benefits subscribers will miss if they don't do it.
For example: "Don't miss out on future subscriber-only offers. Add Sender@XYZ.com to your address book or contact list."
Create a reply campaign.
Most e-mail clients will either automatically add your sending address to the sender list if the subscriber sends you e-mail, or it will present the option to add the address. Instead of the lame and off-putting "Do not reply to this e-mail address because we won't read it," turn your reply address into a place subscribers can ask questions, comment on products, or even enter to win prizes.