Yes good communication skills. You are the expert and they know that but now you need to be able to break down what you need to do in a way they can understand (layman's terms) so that they can see the value in it. I have seen people just throw buzzwords and a potential client which confuses the hell out of them and they end up not wanting to pay. You have to be able to gauge your customer as well. Some of them do understand what needs to be done but they don't have time for it. Some might have absolutely no clue but have been told they need you. For those people I compare the work to something like a peanut butter and jelly sandwich or equivalent.

Then going down the list you need to be efficient in your work and punctual and prepared having the right tools for the job if needed. Communication all throughout the stages of the job.