One of the most invaluable tools I've learned regarding customer service is to utilize your ears. I was listening to a series of tapes about customer service and in one scenario a woman had contracted for a party at a hotel. It was disastrous and she was justifiably angry. The hotel manager took her aside and let her tell him everything she was angry about, quietly listening and taking some notes. Her tirade went on for about 15 minutes and he patiently and quietly nodded and listened. When it seemed she was just about finished, he calmly asked her "And what else?" She looked at him, thought for a minute, and took a deep breath and said "Nothing". It completely diffused her anger and once she was no longer angry they could begin open dialogue about the real issues.
It was very effective on many levels, The manager allowed the customer to completely vent, say everything that was bothering her and gave her his complete attention, making eye contact with her (which is very important when face to face with someone!). He didn't try to correct her, didn't try to address any of her concerns, just allowed her to complain as much as she wanted, nodding and listening. Even asked her for more, which gave her the opportunity to think about it, and make the statement that that was all, which is huge.
I've found that although no one likes to be yelled at, complained to or bitched at, we're in customer service situations all the time and we are going to get the angry end of the stick in many cases. Its better to address things up front and get them out in the open, allowing them to vent their frustrations and assuring them that you care and are there to help. A few minutes at the beginning spent listening to them is well worth it in the long run, as it sets up the whole tone of your time with them. They will know you care about their issues, will take the time to listen and allows them to completely dump out their frustrations at the start, (none of which are personal toward you, by the way!) opening up a fresh attitude toward you and the job you're doing. At the start of the job tell them that you want to take a few minutes to go over their concerns. Sit down with them let them know that after you start you're going to be busy and might not have time later to go over things when you're busy working, and that you want to take a few minutes in the beginning to get on the same page.
Think about how you'd want things to be if you were in their position and how important it is to you when someone takes the time to let you know they're on your side!
Good luck to you!