Adding an arm raise to an already excellent exercise like the plank improves posture and core strength, which makes me feel better, look taller, and feel more confident.
How to do it: Start to get into a pushup position, but bend your elbows and rest your weight on your forearms instead of on your hands. Your body should form a straight line from your shoulders to your ankles. Brace your core and maintain your hip placement as you lift your right arm straight out in front of you. Draw your shoulder blades down and back as you lift your arms. Hold the position for 5-10 seconds and then switch arms.