Here are some of my best tips about building your list of potential customers!
1) Give away free content! A pdf report is still one of the best things to give way. Keep it short - 5 - 10 pages is alright. Business people usually do not have time for long-winded ramblings.
2) Make sure the title of your ebook solves a very clear problem. Don't make the mistake of giving away an ebook that is general. Make sure it solves a problem or it gives very useful tips. Here are a few Examples.
3) Use an email marketing software to keep in touch with the people on your list. I think MailChimp is still the best. It's free for the first 2000 people on your list.