So you scour all the marketing books and Internet articles, and you talk with others who have gone before you to see if they have a secret.

Then, one day, you take the plunge. You begin to market your freelance business. You come to realize that if you don't aggressively market, you are going to get nowhere fast. And that's not why you became a freelancer. You became a freelancer because you wanted to call your own shots and you believed you could make more money working for yourself than for someone else.

If you are diligent and determined, your marketing efforts soon begin to pay off. You land a couple of clients. One of them, in fact, may be a big one. It's a large company with tons of work for you to do and plenty of money to give you in return. Best of all, you've managed to impress them.

Welcome to the gravy train!

Well, not so fast. It is extremely tempting to climb onto this gravy train and go along for the ride as long as it will take you. But "putting all your eggs in one basket," or devoting most of your freelance work to just one client, will end in disaster…sooner or later.

While you are so busy with your big client, you tend to curtail or even stop marketing, so no one else ever learns about you. You eschew smaller clients and projects because you don't have the time and you can make a lot more money with your big client.

Worst of all, the day will come (I promise) that your gravy train will derail. The company will be swallowed up by some bigger, impersonal corporation that doesn't know you and doesn't care. Your job will become so important that the company will decide it needs a full-timer instead of a freelancer. Or your job will become obsolete as the company moves in a new direction. The contact people you have so carefully cultivated over time will gradually move on to other jobs, so you will have no one on your side. Or, heaven forbid, you will make an egregious mistake that just cannot be forgiven…and you will be shown the door.

I know this from firsthand experience. For more than 12 years I rode the gravy train of a company that handed me thousands of dollars every month. Then things began to change. The company is no longer operating, and my contacts have all departed for parts unknown.

Too many freelancers make the mistake of allowing a large percentage of their income to come from one source. If one client supplies more than 20 to 30 percent of your revenue on a consistent basis, you need to find more clients and other sources of revenue…as soon as possible.

I've also known people who worked full-time and then decided to freelance almost exclusively for their former employer! Same mistake. Don't let familiarity doom your bottom line. Always be on the lookout for new clients.

(One caveat: In the very beginning of your freelance career, you may have only one or two clients to start, so they will, of course, contribute a large percentage of your income. But after about six months, you really need to have multiple clients and many baskets full of all those golden eggs.)

In a previous article, I mentioned that the Number One mistake freelancers make is turning self-employment into an excuse for self-indulgence. Having one big, lucrative client is a surefire way to fall into the trap of self-indulgence. Self-indulgence leads to all kinds of excuses for not working. And when you're a non-working freelancer, you are a broke freelancer.

While it's okay–and quite wonderful!--to have a big, lucrative client, don't let that client dominate your time or make you financially dependent on him or her for your livelihood. Unless, of course, you'd rather quit freelancing and go back to work in a cubicle with half-hour lunch breaks, constant pressure, few if any chances, a dismal destiny, little prospect for advancing in your field, and no chance to make more money–no matter how hard you work.