Ask HN: What strategies do you use to encourage users to promote your website?

What strategies do you use to encourage your users to promote your website? Here's a bit more about my specific situation;

I created a saas product used by schools some time ago. I don't store or collect the email addresses of my users except the initial school contact. Therefore I have one contact for roughly 80-100 teacher users per school. I get really positive feedback from my users and I would like to harness this without getting in folk's faces too much.