If you start your opt-in list, better read this before you go any further:
http://www.ftc.gov/bcp/conline/pubs/buspubs/canspam.htm

Because of enormous spam email flooding the inboxes, subscribers are losing sense to differentiate between legit or spam email. They tend to click on 'Mark as Spam' button so often.

So you, as a legit email marketer follow some priciples that keeps you in a position to defend yourself:

1. Strict double opt-in list. Your subscriber gives you the permission to receive email from you.

2. Get subscribers details during subscription like ip, date and time, url of subscription form. So you can have the proof that they willingly subscribed.

3. Keep unsubscribe link in every out going email. Give them option to get out of the list if they want to.

4. Display your physical address in the emails. Let your subscriber know that you are not a fake.

5. Don't put misleading information, quick rich plans etc in your emails. Send good content.

6. Keep an abuse/spam complaint form on your web site.

7. Always be available to your subscribers through your helpdesk or online contact form.