Like Hodgeman said there is no sure fire way of doing this on iOS. It also depends on how many backers you think you will need to provide the app to and if you want to give them the game before or after release.
We currently have an enterprise apple account so it means that we can distribute pre release builds of our app to beta testers and high profile clients but, for a kickstarter game this is probably too expensive for you.
Apple have just announced at WWDC that they will be integrating TestFlight in iOS8 so that you will be able to have up to 1000 beta testers who can sign up just by giving you an email (there is no longer any certificate to install).
Other than that there are promo codes but these are very limited and you will probably want to save these for reviewers. Or you can send somebody a app store credit gift the minimum in the UK is now £15 so probably worth more than you are pricing your game.