Become an expert in a given field. Have great professional and people skills. You are not really going out there looking for clients you're going out there to meet people and let them get to know you. People do business with people they like and trust. And, you've got to be good.
Tenacity is the name of the game.
Never give up. Don't take rejection personally. Remember, the easier you can make things for your clients, the more likely they will be to use you. Just career exploration.