Alternate arm and leg (bird dog) exercise

Do this exercise slowly. Try to keep your body straight at all times, and don't let one hip drop lower than the other.

Start on the floor, on your hands and knees.
Tighten your stomach muscles.
Raise one leg off the floor and hold it straight out behind you. Be careful not to let your hip drop down, because that will twist your trunk.
Hold for 5 seconds, then lower your leg and switch to the other leg.
Repeat 8 to 12 times on each leg.
Over time, work up to holding for 10 to 30 seconds each time.
If you feel stable and secure with your leg raised, try raising the opposite arm straight out in front of you at the same time.