Simple CGI scripts will allow your visitors to recommend your site to their friends directly from your web site, and another script could be implemented which allows visitors to email your articles to associates just by entering an email address and clicking a button. All links CGI script as an example. Here's another idea. Give them free copies of your products for their own personal use and many will be more than happy to take you up on your offer. Aside from just providing great content, see if any of the following apply to you.
Flattery works great.
Don't just let your visitors enter themselves into your contest, allow them to enter a friend as well. Just say that if they are a winner both people will get a prize, and allow them to enter their friend's name and email address as well as their own. That's more free exposure for you. This works extremely well, especially if your costs are low. And there are literally countless ways to go about doing it.