If Im not mistaken, the serratus anterior works synergistically(together) with the upper and lower traps to provide mobility and stability during shoulder elevation.
Exercises such as behind the neck press, front raises, military press, arnold press, incline shoulder press should serve to work the serratus anterior. A good body weight exercise for the serratus anterior would be a push-up(specifically deep push-ups into the negative).
Here is a good link:
http://www.ncbi.nlm.nih.gov/entrez/q...&dopt=Abstract