Training your abs and obliques is important, as it strengthens your whole mid-section, which helps to reduce back pain, stabilize your spine, and minimize your risk of injuries. When many people think of ab and oblique exercises, they immediately think of sit-ups and crunches; however, these can cause stress and pain on your neck muscles, so you need exercises which focus on these too.
If you still wish to do sit up-type exercises, then a far better option is the curl up, which doesn't require you to strain your neck and upper back. Lie on your back with your legs bent. Suck in your stomach muscles and start with your head and shoulders on the floor, then lift them up by contracting your abdominals. Pause for a second in the top position, then lower yourself again. Do 20 repetitions. You can change your leg position by extending one leg and keeping the other one bent. Spine specialist Dr. Stuart McGill recommends placing your hands under your lower back to prevent any excessive spinal arching.
While the curl up trains the movement function of your abdominals, the plank is an excellent exercise for training their stabilization function. Lie on your front, and support your bodyweight on just your forearms and feet. Aim to keep your hips and torso in a straight line by keeping your core as tense as possible and stopping your lower back from arching. Hold the position as long as you can with good technique, rest for a minute, then do it twice more. Aim to increase the duration of your planks over time.
The side plank is similar to the regular plank, but places more of a focus on the oblique muscles and the side of your mid-section. Lie on the floor again, but this time on your side. Place your left forearm and left leg on the floor, with your right hand in the air, and right leg on top of the left. Push your hips as high as you can off the floor, and hold the position for as long as you can again. Follow the same sets, reps and progressions as for the regular plank.
Wood chops target the stabilizers of your abs, as well as the movement function of your obliques. Attach a resistance band around a sturdy upright structure at head-height, and stand to the right of it. Grab the band with both hands over your left shoulder, and forcefully bring it down towards your right knee by bracing your core muscles and twisting your hips. Return to the start position under control, and do four sets of eight reps per side. You can also do this exercise with a weight plate, or using a cable machine, and perform it the other way round -- lifting the weight from bottom to top.