I would also add that you document as much as possible.
Clients requirments. Design. What is included in the price. For instance how long will you provide support, etc. etc. Keep all communications and write up what is agreed in meetings and send copies out.
It is amazing how much comes up when you actually write something down in detail. Also the client will take it more seriously and think you are more professional.
Finally - later on when you have both forgotten the fine details it can be referred back to and any disagreements hopefully resolved amicably.