I have given my business card to multiple other psychiatrists, primary care docs, therapists, and patients who have kept it and send me referrals by looking at the phone number, email, and website on my business card. They've explicitly told me this.
I splurged on nice business cards but even then, it's $0.50 per business card. It's paid back itself in droves. I've probably paid maybe $75-100 this past year on business cards, which pays itself back >10x with just one intake from them. I agree with dl2dp2 that it is the communication that is probably the best though. The business cards is just to remind them how to get in contact with me.