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.
4) Use Whatsapp or other tools to text your list from time to time.
5) Remove those who are not opening your emails from the list from time to time.
6) Make sure your emails always contain useful information that your readers will benefit from.
7) Don't worry about people unsubscribing. You cannot please everyone. Your product cannot serve everyone equally. Let other people serve those who want what you cannot provide.