When I sold services in a business-to-business environment, I adopted the philosophy that I would treat each client as if he or she were my one and only customer. That didn't mean I was available for conference calls at 2:00 a.m., nor did I do any illegal favors... but I did make sure that when I spoke with someone, I was truly focused on what his or her company needed.
By being attentive, I found that it was easier in the long run to iron out any rough patches. (And believe me -- even in the best of partnerships there are stumbling blocks.) You see, the clients really felt that we had a rapport. And we generally did.
Of course, I didn't enjoy working with all my clients. In fact, there were a handful who drove me to the brink of wanting to become a hermit on some remote island. But they never knew.
Today, I employ that same "you're my only client" philosophy when dealing with freelance writing customers.
It's not easy.
There are times when I'm tempted to say to a particularly demanding individual, "Hey, listen... I have other clients, you know!" But as long as my customers treat me with an element of respect and professionalism, I never mention the other projects on my plate in direct terms. Occasionally I'll refer to my "workload", but that's all.
Now, I realize that there are times when it makes sense to break this rule. If a client acts as though I'm lower than the dirt under his or her fingernails, I'm certainly not going to bend over backwards. I'm not a doormat and I don't suggest anyone take on that identity.
That being said, the next time you speak to a client or prospect, I encourage you to focus all your attention on his or her needs. Be incredibly and genuinely attentive and get to know him or her just a tad bit better. Even if you're only corresponding with your customers via email you can still develop a strong relationship by asking relevant questions and answering your mail within a few hours (if possible.)
In the end, your client will feel like your "one and only". And you'll both reap the benefits.