Speak to them on their own level without making them feel stupid(takes practice). Most people don't even know what a 3 way switch is, they think it means 3 switches in one box. If they don't really understand what you are talking about after they hang up they will feel uneasy about what it might cost them. Show sincerity that you are really concerned about their problem/project.
If there is a safety issue I always make them feel as if I am putting them ahead on the list even if I'm really not. Most of all, show up when you say you will.
