it depends on the type of shoot and how much time you have. if you are talking about personal/family portrait sessions, take 5 minutes to chat before diving in. ask questions and listen. if it's a couple, ask how they met, what's their favorite place to hang out, where their from, where they live. if they are from or live in a place you're not that familiar with, get them to give you the names of their favorite places to eat. take some time to find some common ground. ask them what's the silliest thing they've done together (that they can tell you about).
also, take a moment to run them through the process. tell them we're going to start with some awkward ice-breaker shots, then move into the more comfortable shots, and wind up with some spectacular high-personality shots.