Don’t only over-deliver when you sell a product for money, you can also do it when you promote something that is free.
When you promise something in return for an email address, don’t just deliver what you promised, add a surprise bonus. Selling and delivering the free stuff need the same attention as paid items.
Try a new way to make the opt-in sequence convert better.
Let them read (by simply clicking) the first page of what you promise them in exchange for their email and only then ask them to opt in. This, of course, should sell the whole product you are giving them for free.
An experiment for a new way to get better conversions.
Instead of promising the visitor something in exchange for their email address (like a free report), simply promise them a personal connection with you – whenever they reply to an email you send them you will personally (or another human and not an automated system) always reply quickly.
Get people to opt in right after they had a great experience at an event (concert, theater, lecture, etc.)
Print your message (why they should opt in) and ask for the details you need (email address, name, etc.) on the back of the ticket, and have collection spots on their way out where they can drop it off.