Gotta love wiki
"All three parts described above pull the scapula forward around the thorax, which is essential for anteversion of the arm. As such, the muscle is an antagonist to the rhomboids. However, when the inferior and superior parts act together, they keep the scapula pressed against the thorax together with the rhomboids and therefore these parts also act as synergists to the rhomboids. The inferior part can pull the lower end of the scapula laterally and forward and thus rotates the scapula to make elevation of the arm possible. Additionally, all three parts can lift the ribs when the shoulder girdle is fixed, and thus assist in respiration. 
The serratus anterior is occasionally called the "big swing muscle" or "boxer's muscle" because it is largely responsible for the protraction of the scapula — that is, the pulling of the scapula forward and around the rib cage that occurs when someone throws a punch.
The serratus anterior also plays an important role in the upward rotation of the scapula, such as when lifting a weight overhead. It performs this in sync with the upper and lower fibers of the trapezius."
So it sound like push up pluses like Dmunnee mentioned would be a good bet. Also try weighted pec minor dips with a forward lean (http://www.youtube.com/watch?v=IerK6jDwJN0
), as well as being sure to add standing over head press.