A few quick tips (mostly free!)

1) Put a sign-up box (just name and email) at the top of the home page. Offer a special report or newsletter sample.
2) Add a forwarding call to action to the emails you already send out. For example, “Did a friend send this to you? Claim your own copy here (it’s free).” Or, “Know someone else who would like this newsletter? Send it to them here.”

3) Advertise in other newsletters aimed at your target market.