Preventing oneself from becoming this "tool," became an investigation into design diplomacy. More often than I'd like to admit, a design project would go downhill fast due to lack of this communication in the early stages. This has actually been illustrated (within a disturbing degree of accuracy) by The Oatmeal in the comic: How a Web Design Goes Straight to Hell.
These key action elements can help prevent communication breaks, and improve client relations while designing with the varying politics and company cultures that hire you.
Setting the tone with the decision makers: From the very start, make sure you and your clients understand that they are hiring you because you are an experienced professional that knows design, and that your job is to give them what they want, as well as what they need from a design. This is their project, and that your ego is nowhere in the scope of their logo/website/design. This is only effective if you are also consistently communicating with the people who will be making the decisions when presented with drafts. If you're communicating with focus groups, and submitting drafts to a collection of staff who don't have any "pull" in the final say of the project, your efforts will be wasted, and your perspective will be skewed to an unintended audience.
Hold the reigns while keeping the committee involved: It's very important that the client, and all involved with the outcome of the design, feel that they are being heard and understood as a part of the process. This engagement encourages trust from within the company, as well as valuable feedback and perspective from multiple levels within the organization. If at any point the project scope starts to creep, or the primary goal of the design starts to lose focus, it's up to you to gently remind your clients why this is happening in the first place, and be able to back it up with data to state your case firmly. Practicing these qualities will help build and maintain trust in your experience and abilities. This trust cannot be understated.
Prepare your deadlines for the unexpected: Unforeseen anomalies can pop up at any given time, and often do so when least convenient. In addition to unpredictable hurdles, creativity is a difficult resource to regulate and control. There will be times where it is very difficult to get any new ideas worth giving to a client, and that creative block can last for a long time. When estimating deadlines, it's a good rule to take the amount of time that it would take you to complete the task (if you were to sit down and work on it straight through), and multiply by three. A lot of time can be lost in scheduling with other collaborators, administrative tasks, research, and brainstorming. Not to mention technical difficulties and prioritizing other projects. If you work at all efficiently, you'll be able to not only meet the deadline you set, but over-deliver, and clients always like getting what they asked for before they expected to get it.
A neutral voice and an open stance: Keep calm, and use active listening. If you're ever in a difficult situation where your client is not happy with a project, the worse thing you could do is to let your emotions escalate in response. Use active listening techniques like asking questions to clarify, or paraphrasing what your client is saying in a calm, neutral tone. This slows down the conversation enough to give you time to think about the problem, it makes the client feel that they are being understood (not just heard), and it helps reduce the chance of miscommunication.
If these conversations occur in person, make a serious effort to have engaging body language. Maintain good eye contact, lean slightly forward, actively remove any distractions, and be committed to understanding what they are saying. Try to refrain from having a scowl or frown on your face. Smiling might not be very appropriate, but don't project yourself as angry. Present yourself as understanding, sympathetic, ready to solve the problem, rational, and eager to please.
Knowing the importance of clear and diplomatic communication, client interactions don't have to be arduous battles between egos and educational backgrounds. Once trust is established, the conversation can take a more reasonable approach to making sure everyone gets what they need, and the client leaves smiling.