You want to think about it from your user's perspective: what level of product development do your potential backers need to see to trust you and believe in your product? That's the minimum level of development you'll need prior to you starting a crowd-funding campaign. Yes people can steal your idea, however there are lots of people selling snake oil out there (falsely advertised products)... If you haven't built a demo or prototyped or what you claim can't be accomplished easily... then the internet will smell that it's a scam, and you'll loose trust with the potential users. Thus, in terms of product dev think about users.

In terms of risk mitigation (people stealing your idea) with crowd-finding, it's part of the consumer game. So, figure out a good strategy to keep the "how" protected via patents or vague enough so users will trust you and copycats will have major hurdles copying you. If you can't do this for your particular product, have everything else in place to ship so you can easily seize first mover's advantage.

Source: we just performed two successful crowd-funding campaigns on kickstarter... and we love soma. They documented their process and opened up a lot of their resources. I'm a fan of their work.