Know what their expectations are, make a plan, and re-set expectations as needed… before the shoot.
One morning, I was packing up gear for a kid shoot. When I started packing, I suddenly realized that I had no idea what kind of pictures they wanted. Should I take my DSLR and my strobes with light stands and umbrellas? My lightweight Fuji for running around after kids? Guess I should take it all, just in case. For some reason, when planning the shoot, it simply didn’t occur to me to ask how many kids there would be, what their ages were, or ask what kind of pictures they were hoping for. A lot of stupid things to forget.