Denise Cox lists these 10 ten elements she thinks are essential for your opt-in forms:

1. The ability to sign up should be on every page of your website and your emails.
2. Samples of past issues.
3. Give subscribers as many choices as you have available, e.g. frequency, monthly mailings AND event alerts, or just monthly, etc.
4. Tell people very specifically what they’re signing up to.
5. Tell people how often they’ll hear from you. Keep that promise.
6. Easy to sign up / sign off.
7. Test the form to make sure it works!
8. Let people choose between html and text and maybe even a mobile version.
9. Add a link to your Privacy Policy: tell people very clearly what you do with their data and the tracking information you collect.
10. Once they’ve signed up, send a note acknowledging that they are now on the list.

Source: Denise Cox’s blog