Act, but do not react.
Kids know how to push our buttons, don't they? Reacting to negative behavior can actually reinforce exactly what we don't want to see.
Reacting means acting instinctively and without thought. A natural response to bad attitude or sullenness can be frustration and anger...but unfortunately, this only creates more problems, if not immediately then certainly in the future.
Acting means thinking first and then acting in a way that will achieve a positive result for you and the child. Cooling down and giving yourself time to think demonstrates a coping mechanism that the child can mimic.
For more tips on managing challenging behaviors, check out this quick list of alternatives [PDF].