Send out emails to your users every once in awhile to engage them (every week).
Customers love newsletters, but not the kinds that they see as spam. The best kind of emails are ones that are event based. Event emails have 50% open rates (normal emails have about 20% open rates). Event driven emails are basically emails that are tailored to a customer's preferences or profile. For example, birthday emails sent to the customer on the customer's birthday is one. Sending a customer that hasn't logged on for a long time is also one. Asking a user who's finished 50% of his/her profile to continue on the project is one.
An example of an event email is:
1. "Thanks for signing up for StartitUp. The next step for you would be to start your project. Start a project now! If you have any questions, feel free to shoot me an email at email@example.com."
2. "Dearest John, we noticed that you were not uploading photos into your account. Please come back so that we can stop missing you!"
See here to learn more about event emails: http://www.bootstrappist.com/archives/the-rise-of-event-based-email-marketing/
Email newsletter tools you can use: