Starting business as sole proprietorship/freelance

I want to start a sole proprietorship as a freelance web-designer/developer doing some work to various clients in a wide variety of indutries. I plan to use WordPress mainly, but will do other stuff as well. I’ve seen many freelancers around the internet that are doing well in the business, some of them I met at the Wordcamp. What do these guys do to get there? Do they start a sole proprietorship and stick with it and make good money just like that? I’ve heard of guys making close to $100k a year as freelancers. Is a sole proprietorship enough to achieve this or would I need to turn into a company in the future?