The sit-up is a great movement that you can use for abdominal development. It doesn't require any equipment and it can be done anywhere!
Ensure that your feet are in the butterfly position (feet are together and knees out)
Doing so completely takes your hips out of the equation so that you can isolate the abdominals.
Start with your back and shoulders flat against the ground.
Squeeze your abs and pull forward to rise into the upright position.
Try to bring your shoulders in front of your hips at the top of the movement.
Touch the ground with the tips of your fingers at the top of the movement to maximize the range of motion.