By concentrating the power into one part of your hand, it becomes much easier to control the clay.
This applies particularly to centering which is when you need to use the most power to get your clay in the middle of the wheel head. After coning the clay several times, I use the heel of my left hand and imagine I'm pushing an invisible line right through the centre of the ball of the clay, whilst applying a little pressure with my right hand on the top of the clay. Although the whole of my left hand is wrapped around the side of the clay to control it, it's very much the heel that's doing the work. By concentrating the power into one part of your hand, it becomes much easier to control the clay.  Put your body power behind it (rather than just the power in your hand alone, which isn't very much), which is easy because your arm is connected to your body... see tip 1!