Include Opt-Ins On Order Confirmations

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Making optins a part of your purchase or download process is just
plain common sense. Always include a check box for subscribers to tick
off the option into your newsletter on order and download confirmation
pages. Make this easy, and again, clearly set the expectations of what
they will be receiving.
Include opt-ins on order confirmation

If they didn’t opt-in at check out, or even if they did, hit them
again on the order confirmation or other services or transactional
message. There are technically two approaches here. The first is harder,
which is to present this option dynamically only for non-subscribers.
The second and easier approach is to include the opt-in message on all
outbound service related messages and simply suppress for duplicate
addresses when you mail.

The issue with this second, easier approach is that subscribers may
think you are not in tune with them. When you present a generic offer to
all transactional email recipients, they may not think you have your
act together, or they may fear they will get more email from you.
Although, it’s more elaborate, the first option ensures your list will
be free of duplicated addresses and removes the risk of suppressing
addresses unnecessarily. Addresses that tie back to a profile might have
richer demographic or behavioral information that purely an email
address.

This is just one quick tip to expand your lists quickly and allow you
more options to build subscribers, especially by cross-publishing to
different lists.