As with everything else you have to implement, test and perfect strategies that work.
These days online you can write, create and market a FREE EBOOK then you can target prospects using the ore inexpensive "2nd Tier" traffic - or - use direct mail.
Check out the following link FREE EBOOK
The easiest way to build an email mailing list is by offering a FREE EBOOK.
If you are using direct mail your marketing funnel should be substantial. If you are using Tier 1 advertising like Facebook or Youtube or Google because of the expense your back en marketing must be ready and working.
Another way is to embed within the ebook, and on the "thank you" delivery page a related offer of your own or an affiliate offer.
Let's go through a scenario - suppose you have the appealing FREE EBOOK offer and you are using direct mail - or - you are using Tier 1 traffic and advertising. Either way you might expect to spend upwards of $10 to $20 per opt-in.
If you add to the mix an immediate related offer - your costs per opt-in can go own to zero.
Remember,"break even" is a sure fire win in any marketing campaign including direct mail. Why? The value of the list that is built. It is often state that building an email list can ad $1 - $2 per month in revenue - so if you add 30,000 opt-ins you can pretty much expect to make $30,000 per month in additional revenue. At that point who cares if you "just broke even" on the initial offer.