This exercise works your muscles better with weight.  I often use old cat litter containers filled with sand to do this exercise, to utilize weights on a budget.  Any object will suffice, even a water bottle.

Start in a plank position, on your hands and toes.  Have your weight at your hands (you can use only one weight and switch back and forth, if you want).  Pick up weight, and keeping your elbow as close to your body (proximal) as possible, lift weight to your chest.  Lower weight and pick up weight with your other hand.  There should also be a focus on your hips and back with this exercise as well.  Try to keep your hips as still as possible and keep them from tilting and swinging from one side to the other.  One way to advance this exercise AND work on your form is to lift the opposite leg as you are lifting the weight.

Perfect practice makes perfect performance