If you want your freelance writing venture to be successful there are a few important rules to always remember.
While the cliche' says "It's easier to keep a client than to find one," it can sometimes be difficult to do if you take your relationship for granted. While you might have lots of enthusiasm for your first project, it's easy to lose passion and not be as thorough in subsequent projects. Don't make that mistake. By taking your time, focusing, and doing your very best on each writing project you'll have a much greater chance of keeping the client long term.
Don't take criticism personally. If your client wants a revision made, it is probably not a sign of poor quality work, it may just not be what the client wants. Making money using your writing skills always means providing the client with what they are seeking - not necessarily how you would write something yourself.
Customer service is crucial in any good working relationship. Always respond quickly to emails and other messages from your client. If you leave them hanging, they will look elsewhere. Being professional at all times is also a must. Make sure you can meet the deadline that is set before accepting any assignment. If an emergency arises, offer to refer your client to another writer who can help if at all possible.
Proofread, proofread and proofread again. Misspellings and grammar errors will make you look like you don't care about your work. We all make them, but it's important to spot them before you send the finished project to your client.