Don't be shy. Go out and meet people. Show your work around and try to strike alliances and rapport with ad agencies, graphic design studios and other illustrators. Seek out informal and formal media networking events. Always carry business cards with you when you leave the house! ... something I often fail to do, and later regret.
Find a mentor. Find multiple mentors.
Reach out to fellow illustrators and ask questions. We're generally a friendly bunch. Most, if not all, chose this career for ourselves. I don't think many people fall into illustration by accident. You'll find most are happy to talk about their experiences and relate advice.
Promote your work.
Send out emails, postcards, portfolios and samples - and be creative! to stand out from the glut they receive every day. They'll get back to you if they like what they see, but it's wise to send a follow-up email.
I personally don't make cold calls, but some artists, such as Luc Latulippe, advocate it. In the past, I've found it to be a waste of time, and I fear that I'll come across as a pest. Art directors are usually insanely busy people, and it's rare that you'll catch them with time enough to chat... it's more likely that your call will be breaking their stride.