Every time you tighten up, breathe out. Release all your air, even when working the obliques. If you hold your breath, your muscle will take on a rounded shape, as it tones and tightens around the air in which you are holding. You will not achieve a washboard look. Breathing will also help to ease the pain of the exercise.
Proper form is crucial to exercising, especially when working the abs.
Poor form is not only inefficient, but can lead to injuries, especially in the back. Be sure to focus on the muscle you are working. Visualize it tightening and releasing. Quality and not quantity is the key. It is better to do 12 properly formed crunches than 20 improperly formed crunches.