I once worked as an independent contractor (ad specialty sales) and the distributor wanted me to sign a non-compete contract. I refused since I felt that if I was paying my own expenses and generating business for him (on a 50/50 margin) there was "nothing in it for me". He desperately wanted the customers that I could bring him so he backed off. He had a small customer base of his own clients when I went to work for him. I knew who they were and have, to this day, never called on them.
I realize that as contractor, I would bear my own expenses, but more and more he started relying on me to provide services to him. I bought a copy machine, he wanted me to make copies of stuff. I bought computer, scanner, software, laser printer, fonts, clipart etc. in order to generate camera ready art for my clients. He started billing for my artwork and taking half the charge as his own! He even started asking me to create art for his clients at the same "rates". The final straw was his listing of a toll free number that I was paying for so that his clients could call and leave messages for him!!!
I left him after 5 years and opened my own business. He was livid and tried to tell me I had a no compete clause. I asked him to show it to me, he never could. I gave him one month's notice and took that time to get my business ducks in order. Since my customers were used to calling me on that toll free line, he was SOL. I'd say 95% of them moved with me. From day one I doubled my income and have never looked back.