The following sample program is based on the work of Stuart McGill, PhD. and has gained popularity do to its protection of the spine and its basis in science. While this program is not appropriate for everyone, most young healthy people should be able to perform this program. Do not attempt to perform this program if you are advanced in age or have health problems.
Curl-up
This exercise is intended to work the abdominal muscles of the front of the torso, while protecting the lumbar discs. The key to this exercise is the isolation of the abdominal muscles while avoiding lumbar flexion. Lie on your back with your arm or a towel under your lower back. This is intended to maintain the normal curve of the lumbar spine. Remember that flexion (bending at the waist) increases the pressure within the lumbar disc. One 
knee is bent, the other is straight, alternate legs at the midpoint of repetitions. The exercise is performed by curling the upper back up while maintaining a neutral spinal curve. Avoid jetting the neck or head forward while performing this exercise. Concentrate on maintaining good form. Hold contractions for up to 8 seconds. Build muscular endurance by gradually increasing the number of repetitions.
Alternating Arm and Leg Raising (Birddog)
From the quad stance and while contracting the core muscles tense (tightening them in a comfortable contraction, stiffening these muscles while maintaining a “neutral spine”) lift one leg and the opposite arm to parallel with the ground. Hold the arm and leg in this position for eight seconds. As strength and fitness improves it is preferred to add more repetitions of this exercise rather than holding the arm and leg up for longer periods of time.
The Side Bridge
This exercise utilizes the same core muscle tensing techniques that were mentioned previously. Form a bridge with your body with your elbow supporting your upper body and your feet supporting your lower body. The top foot should be in front of the bottom foot. Stiffen your stomach and back muscles and strive to keep your spine straight. This exercise should be performed on both sides. Hold contractions for up to 8 seconds. Build muscular endurance by gradually increasing the number of repetitions.