Static business
Don’t depend on a few (or even only a single) clients. Even though this article speaks more to a product than a consulting business or freelancers, I think it is essential to always keep in mind not to get zdependent on too few customers. If you look at your freelance business today and say, "I can’t afford to lose that client," you should make an effort to diversify immediately and break free from that client.
And if you don’t follow their money with your effort, an exodus of just one or two big customers could seriously impact your bottom line.