Starting an IT consultant business, need advice!

Sorry if this has been posted before. I am new to TechRepublic. I got on here looking for information from IT consultants. I have been dabbling in the IT consultancy realm for several years on the side. A new customer has come onboard that will finally allow me to make the leap from being an employee to working for myself. I have a couple of different customers who frequently call me and I have enough contacts and a reputation that I believe I can be very successful at this. Scratch that… I know I will be successful at this.
So I am looking for advice from all the seasoned IT Consultants on here. What did you do right or what would you have done differently when you started up? And basically I am open to any advice that any of you have.