For at least half of my consulting jobs, I had little idea how to get the job done, but I accepted anyway. And so I learned quickly, and slowly became a true expert rather than just a self-professed one. This process requires being very resourceful and creative in problem solving. Having a large network of contacts also helps, since the answers you'll need will often be in somebody else's head and rarely be in a textbook. Your personal skills are as important as your technical skills.
Clients ultimately are people working with other people.
People are more likely to hire you if they like you. It helps if you are a positive, outgoing, cheerful, and likeable person. You must also be results oriented and be completely dependable, especially if you want repeat business. Each client must always feel that he or she is your most important client, and that you put his or her interests ahead of everything else. As for work habits, consulting jobs are often long and complex, so you need to be able to pace yourself, schedule realistically, and be able to manage your own time.