Des Traynor runs one of the best blogs on User Experience around. If you're not subscribed, I highly recommend you do. He's consistently pushed out excellent, thoughtful, instructive, beautifully illustrated, useful articles about various aspects of designing a startup's product as well as the hidden bits like analysing user behaviour.
This particular article focuses on how to increase user engagement, and suggests several specific techniques:
- Make a strong impression
- Always show a welcome message (presented in such a way as to start a discussion)
- Gradually expose the depth of your product
- Define a message schedule
- Announce features and improvements in-app
- Talk with customers during trials
Great stuff. Read it.