From my experience (and I'm humbly surrounded by all-stars), finding these people rarely has anything to do with luck, fate, doing the resume shuffle, or a result of pounding the pavement until you find the perfect designer/public relations executive/account manager. It is a result of the following:
Throwing out the resume (after looking at one major detail). Show me, don't tell me!!! I think the resume is the worst form for ever created for telling a prospective employer what an applicant is really about, and I preach this anytime I talk to a business school. I look for one thing on a resume: how long does the applicant stay at his/her jobs? I rarely find the resume rodeo constructive. In fact, everyone here I brought in without the traditional resume dance (i.e. resume is e-mailed, interview is set up, a job offer is made and accepted). It's all been about the network for me. As in ...
Going to my network. There's nothing that makes me pick up the phone quicker than when a local business connection, or even friend/family member that I know and trust, gives me a glowing review about a person, whether in or out of my industry. For me, it is time well spent to sit down with someone like that and learn what they're about and how their abilities and personality may play a role in the agency, and a role for our clients.
Not just looking within the industry. Disclaimer: there will be areas -- such as senior design/art direction -- that absolutely require someone who has been in the trenches. That said, don't ignore the hidden gems out there just because they can't reference agency experience. In our agency's model, we put a ton of emphasis on a prospective employee's work ethic, entrepreneurship, abilities (and ability to learn) and creativity. Those are more important than "X years of agency experience." I'd rather teach a razor-smart go-getter the way we work than deal with agency hang-ups one may have because of their last agency job.
Asking atypical, buy highly revealing questions. Nothing reveals more about a person than asking the following questions. What do/did their parents do for a living? What are/were their parents like? What book(s) are they reading? What are their favorite out-of-the-office hobbies? All three will give you a very solid look inside the person. You will also get a chance to see what really sparks his interest and lights a fire inside him and how that matches with your agency. PS: I don't care if someone's mom/dad picked up trash or ran a hedge fund. It's about the way they live/work and what they taught their child that will surely be emulated and played out by the applicant once hired by your agency.
Creating a learning environment. This is a whole other blog, but from my experience, people leave jobs because they stop learning and growing long before they leave for a bigger paycheck. Of course money plays a part, but show me the individual who jumped ship for more cash and less growth and I'll bet you they lasted less than a year at the new gig. And if you're one of those sorry types, we both know that within a few short months, the money has gotten old, and you're wondering where you went wrong. It's the learning, stupid.
It may not be for everyone, but by following the above, I have had great success in hiring great people. And keeping them.