Think about what factors are important to you. The strength of the department should be high on your list, but you also need a place where you can feel comfortable enough to function well. Make a list of what is important, and then try to find out how the candidate departments measure up. Much of this information should be available on the Web. It is becoming quite common for admittees to visit various schools before making a decision.
Some departments even pay for part of the expenses of the visit for their top candidates.
Let the department know when you plan to arrive. Arrange to tour the facilities and talk to one or two professors and a few graduate students. Wander around on your own, too. Visit the computer labs, the library, the bookstores, etc. Get a feel for how well the department and the school function.