I'd be interested in peoples idea on marketing to existing customers. Leaving aside the issue of E-Mail marketing, what other strategies do employ to market to your existing customers?
I think regular communication with your existing clients is more important than how how you communicate with them. You can stay in touch with them with the help of email newsletters, mailings or a blog. I would say email marketing is the best way to market to existing customers, your mailings won't be considered spam if they have agreed to recieve mails from you at the time of sign up.
Mailing Lists. Make sure you dont always email them , it can confuse them, make emails simple and easy
Blogging and RSS feeds is about the only other way I can think of besides email. There are the proprietary systems that will deliver desktop ads/messages, but they're not very widely used. It's not like you can Skype, MSN or phone all your customers individually.