One such exercise is the bird dog, a move helps you engage the deep abdominal muscles (the transverse abdominis) as well as your stabilizers to keep from tipping over. I like this exercise because anyone can try it, no matter what their fitness level is. There's a definite balance challenge the first time around, so you may want to practice by lifting one arm at a time, then one leg at a time and then progressing to lifting the opposite arm and leg.
This is a great exercise for beginners who are just getting to know their abs and need to learn how to brace the core for different movements. This is also a great move to warm up before going into more difficult exercises.
Do it right - Begin on hands and knees with the back straight and the abs pulled in. Lift the right arm up until it is level with the body and, at the same time, lift the left leg up and straighten it until it is parallel to the floor. Hold for several seconds, lower and repeat on the other side, this time lifting the left arm and right leg. Continue alternating sides for 1-3 sets of 10-16 reps. Modify by lifting the arm and leg a few inches off the ground until you're able to balance.