Make it easy for people to choose to sign up for your email program. Direct registration (signup) links on your website are a great idea. Drive customers to an electronic process as often as possible. (The less paper involved, the fewer typos and weird stuff, and the better your deliverability will be.) Avoid co-registration or data buying or selling. Like with append, this kind of data is always so dirty that it will undermine your best efforts at maximizing your inbox reach.
Don’t assume permission; ask for it. You can end up with a lot of spam complaints (and even potential legal liability) if you don’t adhere to best practices. Have an opt-in/opt-out checkbox during the registration or check-out process. Don’t pre-check any checkboxes. Don’t assume that everybody who has registered is opted-in, if you haven’t asked them.

Know the difference between transactional messages and commercial messages.  When in doubt, consider a message to be “commercial” in nature, and comply with all of CAN-SPAM’s requirements.  Overzealous senders occasionally mis-classify messages as transactional with the hope of sending the messaging to subscribers even if they have opted-out, and so they don’t have to have an easy unsubscribe process linked from the message. Going down this path means you’ll end up pushing repeated email messages into the inboxes of people who don’t want those messages. Complaints and blocking will follow.