Another core exercise favorite of mine is the Birddog. 
It works your erector spinae, lower trapezius, rhomboids, posterior delts, glutes, hamstrings, multifidus and
(in my opinion, one of the most important) the transverse abdominus.

Kneel on all fours with your knees directly under your hips and your hands directly under your shoulders.
Extend your left arm out in front of you while simultaneously extending your right leg out behind you.
Hold for 5 seconds.  Return to starting position and repeat for 4 sets.  Complete the sets and switch sides.

(image Paige Waehner, about.com)

TIPS on form:
Keep your pelvis stationary.  Try not to let your hips tilt when you extend your arm and leg.
Keep your eyes down.  Don't raise your arm or leg too high.  Your body, once one arm and one leg are extended,
should be toe to finger tip parallel to the floor.