There are many variations of planks that I do but I want to talk about the basic plank. A plank is one of the best exercises that help with core strength. In sum the stronger your core is the less back pain you have, the more stable you will be and the better your waistline will be. Make sure that your elbows are directly under your shoulders. Keep core tight (that means pull the belly button into the spine). Evenly distribute wait on right and left feet and keep your back flat (don't stick your butt high in the air).
I always advice 3 sets of these guys and always try and hold as long as you can. I try not to set specific times, but each time you try a plank try and hold it for longer than your last. You need to continually challenge yourself to see change.