Prone and side planks are among the most effective core exercises around. For those unfamiliar with how to properly perform a plank, refer to my video here. You must know how to perform a proper side plank before attempting a plank press. Furthermore, you should clearly understand the purpose of the plank exercises, which is to strengthen your ability to create "super stiffness" between your rib cage and your pelvis. For those unfamiliar with Dr. Stuart McGill's research on the necessity of "super stiffness" for lower back protection and performance, do some homework. I use his core principles everyday, and they are certainly applicable to any athletic endeavor. Visit his website, www.backfitpro.com, for more information.
A great advanced variation of a side plank, and another opportunity to develop super stiffness, is the plank press shown below. Start with a very light weight, since most people's initial struggle is with the stability of the supportive shoulder on the bottom arm. You'll want to be certain that your forearm of the top pressing arm stays at a perpendicular angle to the floor at all times, just as you would with a bent press.