Here's another variation on the plank which had my core and my arms screaming with sweet pain. This is a good, challenging exercise. Give it a try!
Start as if you were to do a push-up. Drop onto your right forearm then onto your left forearm. You will have lowered yourself onto both of your forearms. Keeping you abs tight, navel to spine, push yourself back up, one arm at a time, right arm and then left arm, into push-up position again. Repeat dropping onto your left forearm first. Try to complete it 5-10 times. This is great for your core, obliques, arms, chest, and shoulders.