Here are a few pointers on proper side planks, which are fantastic way to challenge and strengthen your core.
Lift your hips up nice and high and hold them up high--even higher than in this photo! (It was hot out!)
You can do all sorts of variations; if you're comfortable resting on your hand instead of your forearm (which isn't for everyone, as it bothers some people's wrists), lift your opposite arm up to the sky and bring your top elbow to your top knee; repeat without rushing. Or bring that top arm underneath your chest by rotating your torso so your belly button faces the ground.
You could even add a light hand weight to boost the intensity here.
Another option: while you're holding that side plank position, lower and lift the hips. Repeat, getting your hips up as high as possible each and every time.
All are great ways to target the oblique abdominal muscles, which run down the either side of your torso.
Remember to keep your abdominal muscles engaged throughout. How do you do that? By pulling that belly button in toward your spine. Think
See a quick video here.