I find the Joint Venture campaigns work incredibly well for me. Especially the Amazon.com book campaigns.
Additionally, writing articles that are very content driven on a topic specific to my expertise and of interest to my market.
It is not enough to put an eBook offer on a website, you must drive traffic to your site. Article distribution is a great way to do this. In the resource box I offer something of high value that people sign up for at my site(s).
I distribute the articles to various article directories. The more targeted my message the greater response I receive.
Another thing that has definitely increased my opt in list is offering free teleconferences. I also partner with industry experts and we co-promote.
Every so often I do paid advertising on specific sites, directories or ezines. Again, if I am targeted and know my market I get a much higher ROI.
Kathleen Gage