There are few things that you will have to learn, first is the way of thinking is totally different. If you are designing a website and you get a call from client about some new feature in site that he want, your brain starts thinking about the coding and the easiest way to do it, while for a client it’s just a work that he wants to get done. So never show or tell how you will be doing it, just say it will be done in x days at this price. A small tip here, if you can do that work in 1 day, then tell 2 days. Never tell less time to impress client, you will be only creating trouble for yourself later.
Develop friendly relations with clients, like in free time or when you meet him anywhere, just don’t start talking about work. Smile and talk something good, talking about work and worries all time make a person undesirable. Leave with a tight handshake (softly in case your client is a woman) and again a gentle smile on your face.
Things will not always go well, and in case you are struck somewhere and not able to complete task on time, tell about it to your client. Good way to tell about is saying that you are stuck somewhere or having slight trouble doing it but you will manage it. Bad way is making lame excuses or worst not receive phone call or replying to mails.
So with these knowledge, I bet you will be working like a champ freelancer! Enjoy life.