I like to give the opportunity for them to look through the cards before we start. I give a choice between decks, and invite them to pick up the different cards and choose the one that seems appealing to them.
I think that that helps to relax some people, because it makes the cards less mysterious (than if I were just to pull a deck out of a non-descript bag and start shuffling), and gives them more of a sense of control, since they`ve chosen a pack themselves that looks pleasant to them.
Some people have taken the opportunity to ask about a card that makes them uncomfortable while looking through the deck, and then can feel more at ease about it when we start the reading.
If they`re nervous, I try to be upbeat and reassuring, and to chat a bit while we take turns shuffling the cards.
From my own point of view-- I might be nervous if I go to get a reading, not because Tarot is scary (I love it, of course!), or because I`m worried about hearing something bad will happen, but because of the sense of exposure I feel that comes from a stranger potentially being able to peer into my life, and perhaps see things that I don`t like about myself, or the idea that they might recognize things I`m in denial about (that darn ego of mine, gets nervous when it`s under scrutiny!).
I think there are all kinds of reasons a person might be fearful, and it`s important not to jump to conclusions about it, too.
One time, I jokingly told a woman, ``don`t worry, I`m not going to tell you you`re going to DIE, or anything,`` when she seemed apprehensive before the reasing, and she responded that she had come to see me because someone close to her had recently passed away, and she wanted to know about the changes that were happening in her life as a result. !Whoops!! I felt awful and apologized profusely, and have never brought up the death card pre-reading again, unless the client specifically mentioned it...
I had another regular client who was perpretually nervous, because she was a very strong Catholic having a major faith crisis, and was worried for her soul every time she came to see me, but kept coming anyways.
Wouldn`t you know it, every single reading I ever did for her, the Devil card featured prominantly. It didn`t matter how many times I talked to her about it being a symbol for her fears, and that it came up because she ties herself up in knots all the time over the things she`s afraid of (literally, the Devil, in this case!), and it stops her from doing a lot of things she`s drawn to, being a symbol for temptation and restriction both. She kept being afraid, and she kept coming back for more readings...