It's a lot of networking. A lot of IDs are people that have spun away from large companies (like Nottingham from GM), or have worked in an ID shop and then gone out on their own.

Like any other consulting, you develop a reputation working on smaller projects and then you start getting approached by larger firms. And if you have a contact on the inside it's also easy to design something on your own and then get a chance to present it to the megacorp.

I've known a fair number of people in the toy design business and it works pretty much the same way. It's just industrial design with a different audience.