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Have a mailing list! Email them consistently during development.

The week of launch you should send the following emails:

Pre-launch announcement: "We're launching in a week!"
Day before announcement: "The launch is tomorrow. Here's how much it will cost."
Launch day email: "We're launched! Sign up here"
Day after email: "You've still got time to sign up."
Launch discount is ending email: "This is your last chance to register"

Use services like Product Hunt, Product Hunt Ship, BetaList, to build anticipation.

On Twitter, FB, LinkedIn, Instagram: constantly send out product updates.