I'd venture to say that upper management support may be the most important quality for success. The introduction and acceptance of human factors engineering as an important/required aspect of the design and development process represents a fairly wide-reaching change in many (if not most) companies' ways of thinking. It is upper management that has the sheer power to implement such a fundamental change.
What exactly is upper management support? In a nutshell, it's support at an organizational level high enough or powerful enough to mandate company-wide procedure and process. Things like corporate statements, goals, aims, etc. are examples of upper management level of influence.
What if I don't have upper management support and I don't see it happening? Don't fret...yet. The fact that you're hired is at least a symptom that some part of upper management supports human factors to a degree. However, the key is to develop the support into something that will improve your situation.
One of the best ways to make people understand the benefits of human factors is to involve them directly in usability testing and let them experience testing and its benefits first hand. This method is further discussed in "Involve team members in the testing process."
This person should be familiar with what you are trying to do and is a true believer in human factors. If this person is not a true believer or not knowledgeable in the field, then you need someone that believes in you and is willing to listen to the evidence supporting the need. You need this person to "open doors" for you.
Understanding the goals and concerns of management, and addressing those concerns is critical in obtaining support and backing. Starting at the top and working down the management chain, personal interviews with individual managers can be performed to identify that person's definition of human factors, what they feel they need in their organization, what are their concerns, what do they think the key to success is for such an endeavor, and who else in their organization should have input.
Quickly follow up on any meeting, phone call or correspondence. A summary quickly back to a person keeps them thinking of ideas and often results in further input from the person.
This means doing your homework. You need to have an idea of the concerns they will have and be able to address them with substantial answers.
Gathering all of this information is only the first step. Analyzing the information and then effectively communicating the way ahead is the key. Make sure the concerns you've identified earlier are covered.
Do now and ask for forgiveness later.* Success is an effective way to convince management; however, you need to weigh the risks (i.e., chance of failing and the consequences). This is discussed further in the "Just Do it" technique in the Visibility section.(link to section which justifies doing it right now and doing it informally)
Use minimal techniques and get quick results.* "Discount usability engineering" methods have been shown to be very effective while being cheap and easy. Again, success is a very effective way to convince management of the benefits of usability testing.
Use vocal key customers as test participants.* In most situations, the customer's opinions are held in high regard. Opening the eyes of customers to the benefits of usability can be an indirect yet effective way to get the ear of upper management.
* From Lora Davenport, "Gaining Approval from the Top Down (Executive Approval)" from CHI 96, SIG: Corporate Pioneers, Lessons Learned: Introducing and Promoting Usability Testing in a Corporate Environment, April 18, 1996