Planks are great for strengthening your back, chest, shoulders, neck and core. They are helpful with improving your posture.
You can do a plank at the top of a push-up position with your hands and feet on the ground, focusing on keeping a tight core and not arching your back. You can also do a forearm plank, which focuses more on the core and back, but takes some pressure off the shoulders. If either of these positions are too challenging you can always drop to your knees, for a modified version.
You want to make sure you keep a flat back, and abdominals engaged. This exercise is mainly used to strengthen your abdominals, therefore that is where this exercise should be felt, not at all in your low back.
Hold this position for 5 to 10 seconds. Repeat 5 to 10 times, 3 times per week.
If at any time this becomes painful, please stop the exercise.