Promote up-sells/cross-sells. You can even set up an autoresponder sequence for someone after they purchase and get repeat customers. Depending on the products you sell, you could offer an upsell, or cross-sell related products. For example, if someone buys a digital camera, you can offer to add a lens, a tripod, and other accessories to their order before it ships. Or, if you sell products that people buy frequently (like food or disposable items, like diapers), you can automatically send them offers for new items when you know they’re about due for another order.
“As my list has grown, I’ve just remained focused on the next metric right ahead of me. First it was just to hit 200 subscribers, then 300 subscribers, then 400 subscribes. Eventually the goal was 1,000 subscribers, then 2,000 etc. etc.  As I’ve gone past 10,000 and 20,000 and beyond I’ve actually started to focus more not on the sheer number but on engagement.” – John Corcoran, Smart Business Revolution
I basically had to download the javascript from their site, figure out how their code was written and then tailor it to how I wanted it. Otherwise, I would have been limited to their templates which I didn’t really like. In addition, the Aweber popup code didn’t work properly across all web browsers and I had to fix this manually. (Updated: This problem has been rectified)
Think about implementing referral links to incentivize your subscribers to spread awareness about your brand, too. Referral links allow your current customers to promote trackable links for your business. In exchange for sending new paying customers your way, you can give them incentives like discounts, coupons, vouchers, cash, prizes or redeemable points.