Most referrals come from colleagues or friends of colleagues, friends, or past clients. The keys to getting those referrals are: 1. Be pleasant to work with. 2. Do good work.
Get to know the people on the client team and treat them well. Make them a valuable part of the project and make sure their voices get heard. People change jobs. If the current project goes well, the person who hired you will have her stock rise within the company, and the rest of the staff will eventually spread out far and wide to other companies who will need design services at some point. Your DNA travels with them. (Not literally. I’m hoping I don’t need to add a chapter explaining that.) When the call goes out to find a design partner, they’ll be throwing your name in the ring.
Everything you deliver on a current project and every interaction you have with a current client is business development. All successful jobs lead to more jobs. And you are never not trying to line up your next job.