This is similar to #1. But instead of writing for a free site, you write specifically for a PAID membership site. Or you provide audios…or videos to the site.
Not as many people will view your content, but the ones who do are PAYING customers. Buyers are worth much more than free subscribers (by a 10 to 1 ratio or even more).
Are you a member of any membership sites? What could you create or already have that you could offer their subscribers for FREE?
Perhaps you sell a related product that fits perfectly in with their theme. You could provide a copy for all their customers.
The same principle applies here as in #1, it has to be GREAT content instead of a promotion for your offer. In fact, it has to be even BETTER CONTENT since it’s part of a paid product.