Be sure to breathe normally. You should NEVER hold your breath while doing ab exercises.
Do not exercise your abs on consecutive days. Allow yourself a day of rest. Resting your muscles (even abs) is equally as importantant as working them. Every other day is generally ideal.
Use your hands behind your head lightly. Resist the urge to "pull" your head which can cause strain on your neck.
Listen to your body. A good muscle burn in your abdominals is one thing, but if you feel uncertain pains in your back and/or neck, do not push it.
For overall health and fitness and to make sure your abs will show their best, mix cardio, weight-lifting and a healthy diet into your lifestyle. A strong core will help you burn more fat, but a strong BODY will burn even more.