This isn't really related to search engine optimization... @apollo23 is right - write a free ebook or give something away which is related to your product or service. If you can't write find a copywriter. Then use aweber or something similar to build your double opt in email address list. The gold is in the list!
I heard one on how to get huge list of email a long time ago, its about by looking at online groups and copy those active emails that are relevant to the niche.
if you use this email to send promotions of a website then its not good.. its just going to end up on spam folder.