Communicate patiently with your clients. Remember, they are not designers, so use their language to get your idea across.
I believe there is meaning and a story behind your designs and works, but if you don't put effort into telling me, how will I fully understand?
Respect the deadline; meeting it improves your credibility and will impact your career development.
People prefer to talk to someone who is presentable, meaning clean and tidy. Stylish doesn't have to mean unclean and untidy, do you catch my drift?
Punctuality is important, it is important to anyone who is professional, which includes designers.
Never get angry during the price negotiation process, it's just business and nothing to do with insulting your work. There is no reason for them to do so anyway, if you really think about it.
Your reputation can be proved by business references or winning awards. Don't forget to win more awards, because it is what customers believe in.
Never, ever, attempt to copy others' designs for any reason at all. It is a death sentence to any designer.