This is a basic tenet that applies to any organization. Given the difficult situations that human factors people must generally work in, the personnel issue becomes especially important.
As a minimum, human factors professionals must have a high quality and level of human factors education. What constitutes a high quality and level of human factors varies with who you talk to. Many people view a PhD as a minimum, while others require a Masters. There does seem to be consensus that an advanced degree is generally required. The requirement of a graduate level degree in human factors generally insures a solid background in research methodology, quantitative background in behavioral data collection. The skills gained with advanced degrees play a big role in ensuring quality research and human factors work which in turn enhances the reputation of the group.
If you have the luxury of hiring several people, hire from a variety of backgrounds (e.g., psychology, cognitive science, human factors, engineering, graphic design, industrial design, interface design, computer science, etc.). By creating a multidisciplinary group, you utilize the strengths of each of the different areas of expertise.
If creating a multidisciplinary group is not feasible (i.e., you're limited to the number of people you can hire) you should hire people with similar backgrounds to the people your group is most likely to deal with. For example, if you work with mostly software programmers, I'd suggest hiring a human factors person with a background in programming. It's a situation where familiarity encourages acceptance. (Unfortunately, the reverse is also true where unfamiliarity encourages resistance). As an anecdotal aside, my work environment is predominantly engineers and as such, all design and development is done by engineers. On more than a few occasions when working with a particular engineer for the first time, I've noticed a significant change in attitude for the better immediately after that person finds out that I have an engineering degree.
The most important personal skills are interpersonal skills and communication skills. Given that human factors people spend as much time "evangelising" human factors as they do practicing it, it's clear why interpersonal and communication skills are important.
Especially in new human factors groups just finding their way, there should be a person who's been through the battles and understands both the political and technical issues associated with the growth of a human factors group.