1. I don't do re-writes, period. This is merely a way for a client to get several articles for free. They reject the first version and then tell you that you need to re-write it so you send them another and so on. I got caught up in this when I first got started. The minute a client starts barking about how the article wasn't what he had in mind (after he gave vague requirements) I cut him loose and cut my losses. These people are time vampires and it begins to affect your delivery to other good, professional clients.
2. GET PAID FIRST. I've been incredibly lucky and have only gotten shafted once, but I've heard horror stories about many who have put in a lot of work only to be completely ignored once the work is delivered. On long term projects, ask for at least half up front, and then have the client review the work that you've done so far to make sure that you are on the right track. Communication is key in making sure that you are both pleased with the outcome. For smaller projects, you should be paid in full.
3. Deliver on time. This may sound easy on the surface, but life and other emergencies can get in the way. I lost my internet off and on in my neighborhood over a period of two weeks. This really put me behind schedule. Have a backup plan (I didn't at the time, but do now) so that you can at least communicate with your customers so they know you are not blowing them off and absconding with their money. Trust is a hard thing to establish over the internet.
4. Don't let rejection deter you. It's often the fear of rejection that keeps most people from making any real money when it comes to being a provider. If someone says your work isn't of the quality that they're looking for, then smile, thank them for their consideration and move on. As has been mentioned before in this forum, quality is subjective and largely based on the subject matter. Sometimes this tactic is used to try and convince you to lower your prices. Often these people will still come back and request you to write for them anyway. If they're still willing to pay you for your services, than your writing is obviously good enough for them at the prices you set.
5. Stick to your guns. Don't let anyone intimidate you into doing work for cheaper than you want or in a time frame you're not comfortable with. There's nothing more annoying than when you agree on a delivery date and then get emails every day before then asking when the content will be ready. I've actually sent an email to two clients who were like this and let them know that I didn't think that I could deliver what they're looking for. I got to the point where the money simply wasn't worth the aggravation and stress.
6. Don't be desperate. Desperation can often cause us to work for less than we're worth or to accept being treated poorly. That's the beauty of working for yourself, you don't have to put up with it. Take emotions out of the equation and just simply make the choice not to work for clients who are not willing to treat you fairly and with respect. Even if you're in a dire financial situation, you'll find that when dealing with difficult and demanding people that you're working for way less than you thought. The amount of time that these people demand can put your hourly wage down to pennies.
If you handle yourself in a professional and polite manner, even when they don't, you'll begin to earn a reputation as a solid content provider that can be relied upon. You're going to hit some bumps along the way, just make sure you learn from them and always strive to deliver solid customer service.