Of the many variations of the planks that are out there, the one I'm loving right now is the plank punch. This exercise is going to work your shoulders, arms, glutes, and, of course, your entire core. Take your time with this one; it's extremely important your nail the form perfectly before you start trying to speed through it!
1-Begin in High Plank position. Your shoulders, elbows, and wrists are in line with one another, and your back is flat, creating a flat line from the top of your head to your heels. Really engage your core and your glutes.
2-Raise your right arm and punch your right hand forward, punching out from your forehead. Keep your core as still as possible, resisting any tipping to the left or right that may occur from throwing off your balance.
3-Lower your right arm down back into its starting position, shoulder, elbow and wrist aligned, and immediately raise your left arm, punching it forward, even with your forehead.
4-Repeat the alternating punches, resisting any tipping movement in your core, for 1 minute. Rest for 30 seconds and repeat!