It doesn't sound like yours is the type to get extremely anxious or goofy, but there is one suggestion I've seen that I wish I had known to tell my own kids. The questions will usually start much too easy for them, and then get progressively harder until they are aimed at kids much older than them. I think knowing this in advance can help some kids recognize not to start messing around when the questions seem foolishly easy, or freaking out when they get impossibly hard.
As aeh notes, kids seem to find the WISC quite fun.
