When working with larger amounts of clay, the best method of centering is to use the heel of the palm of one hand, and either the flat or fist of your other hand. This method also works well with small amounts of clay.
To center this way, brace your elbow against your hip or thigh. Position the heel of the palm of that hand so that there is a straight line from your elbow to the center of the wheel. The heel of your palm will force the clay inward. Use the side of your other hand (or your fist) to force the top of the dome down and into the center.
One of the other advantages of this method is that you can carry a loaded sponge in your upper hand. As you center and the clay dries, you can easily lubricate it by squeezing the sponge slightly without having to move your hand to scoop water or slurry from your bucket.