I haven't done it personally but I know of a few that have. I think it's a brilliant way of doing things though.

Check out some campaigns on kickstarter and indiegogo to see what works well.

Look for Pebble on kickstarter or space monkey.

It's also worth doing some customer development with your audience to see what would work.

It's difficult to be specific but I would think about the following in relation to your questions.

1. Give them something of tangible value.
e.g access to the app, delivery of the product

2. Give them something that people can only get if they pay
e.g early access to an invite only beta

3. Give them an incentive to tell their friends. Reward them for doing so.

As I say, most of the ideas of what will work will come from your target audience.