You don’t, unless you have a lot of money. Lawyers are expensive. Designers (or at least the ones who post on the internet) seem to worry about it all out of proportion to the scale of the problem. Half the threads in here are people asking how they keep companies from “stealing their designs.” American and European companies (and most Asian ones too for that matter) as a rule are not going to invite you to submit something and then steal it once you leave. I’m sure it’s happened, but it doesn’t happen a lot more often that it does. Once it reaches the marketplace someone might copy you, but what can you do? Don’t worry about it. If you’re making something at the high end, a cheap low end knockoff is not affecting your business to any significant degree. And if you’re at the lower end, the key is to deliver value- that’s much harder to copy than a design.