A simple pose, yet difficult is the "Plank" pose. Do it from hands or elbows. Keep your butt flat. Suck up your abs, keep feet about hip width apart with heels actively pushing toward the back wall. Google this or go to YouTube. It's a yoga pose. Start with holding it for as long as you can then work up from there. Also look up "side plank"- can do from hand or elbow.