Reward people. Personally thank them on Twitter and send an auto-response that provides them with more value and information.
Define clear rules. Tell people what they are. Tell people if they change and why, but try and stick to your promises. If you want 3 signups before you give something away… tell them that’s how they get in… and let them in! No one should be begging.
Tell them how they are helping. Let them know, without effort on their part… if their marketing is helping you achieve your goals. It’s as simple as saying “Hey – you got 5 people to sign up! Thanks! Here is a free ebook about building a landing page. “
Keep them engaged. Even if you can’t launch yet keep people informed along the way. Demonstrate progress. Send newsletters with information that helps them achieve their goals anyway… even if it’s without you.