Plank is NOT a pushup. A lot of people like to treat it similarly but there are a lot of differences.
You want to keep your arms next to your chest when lifting. Not out wide.
Lift up through your arms and back. Keep your shoulders away from your ears.
Engage core to keep from dumping down. Tuck your tailbone to elongate spine.
Engage legs and internally rotate thighs to assist in tucking your tailbone and elongating spine.
Reach long from heels through the crown of the head.
Make sure your on your toes. This helps prepare for Chaturanga.
Try these little adjustments to help feel the form for a better, more engaged plank.
Healthy Living Blog
Motivation Monday: Correct Plank!