Why have sit-ups fallen out of favor?
First, because they’re hard on the back because of pushing your curved spine against the floor and also because they work your hip flexors. They’re the muscles that run from the thighs to the lumbar vertebrae in the lower back. When the muscles are overly tight or overly developed, they pull on the vertebrae and can be a source of lower back distress.
Second, planks involve a better balance of muscles on the front, sides and back of the body than do sit-ups, which target only a few muscles.
Remember that your core goes beyond the abdominal muscles—sit-ups or crunches strengthen only a few muscle groups.
How to do a plank properly
Start out supporting yourself on the floor with your toes and forearms supporting your weight as above.
Keep your torso straight and rigid and your body in a straight line from your ears to your toes. No sagging or bending.
Relax your head and look at the floor.
Hold the position for 10 seconds to start, over time working up to 30-60 seconds.
There are more advanced variations with a leg lift and with an arm lift.
Source: Harvard Medical School HEALTHbeat, September 13, 2012 About.com Sports Medicine, September 27, 2011