Depending on size of each piece of chicken or a specific recipe, there may be times when you have to use a meat mallet or heavy rolling-pin to flatten chicken breast to even it out.
You always want to put a piece of wax paper or clear plastic wrap between you and the chicken.
Starting in the center and working out to the edges, pound lightly with the flat side of a meat mallet or rolling-pin until the chicken is even in thickness.
Lightly hit the upper and larger part of the breast until it is the same size from top to bottom.
If you have a recipe that calls for the breast to be butterflied this method will insure that both the stuffing/filling and chicken breast will cook evenly.