The plank is a perfect full-body exercise and if you're doing it right you'll be strengthening and toning your arms, core, and legs. If you're usual routine of sit-ups isn't doing the job in helping you get six pack abs, then integrate these moves into your ab work.
As you set yourself up to hang out in a forearm plank be sure to keep your back, abs, and legs engaged. Press your elbows into the ground to make sure that your shoulders don't "wing" out and away from your back and tuck your pelvis towards your nose while drawing your stomach in. Engage the front of your legs while pressing through your heels and you've got the makings of a great forearm plank.
When you feel comfortable with your standard forearm plank, try taking your planking skills into opposition. Begin by pressing up onto your palms from the forearm plank while maintaining the form in the rest of your body. When you feel steady, lift the left foot off the floor as you raise your right arm into the air. Challenge yourself to evenly spread the weight between your grounding hand and foot. As you switch opposing sides try to keep your pelvis even, avoiding any rocking side-to-side as you make the change.
Side plank is great for focusing on strengthening your intercostals and targeting in on those "love" handles. Try to keep your shoulder over your elbow and press the elbow into the floor to keep from sinking down towards the ground. Lift your hips, keeping them in-line with your legs and upper body and feel the burn as your core kicks in! Extra challenge needed? Come up to rest on the palm of your hand, keeping a micro-bend in your elbow.
The variations for planks go on and on, but these three are a good start and will give you the challenge you need to build strength and muscle definition. Try these three exercises for thirty seconds each with a ten second break in-between variations and build up to a minute with ten second breaks. Once you feel like your planking game is strong you can move from static planking to dynamic, moving planks for an added challenge and a little burst of cardio.