You have first offer value. Know their pain points and offer as many free resources, tips, advice that work, etc. first as possible. These can be done through a set of autoresponder emails. Just keep in mind that whatever they opted into, that is the information you should supply.
After a few emails, you can offer affiliate programs that are free that have helped you - again stay on the same subject that your subscriber opted in for.
You can always add a link below your signature as a P.S. - this is gold!
Hope that helps!