To incentivize the prospect to sign up, it’s important to offer them some sort of free bonus, like an ebook, access to a webinar, or whatever attractive freebie you can give them that is related to your business. But you shouldn't have to bust your butt to put this bonus together. You could collect together past blog posts into one PDF, for example.
Solo ads is essentially a method of paying someone to mail to their list for you. I don’t do this very often anymore but it is a very very effective way to jumpstart a list. If you followed along to a previous 30 day challenge I did on my old blog, Business & Blogs, you would have seen me go in to great depth about how I gained almost 1,000 new subscribers through solo ads alone.