Resistance Training Program

Train your lower abdominals first.
Breath out as you crunch up.
If you have your hands behind your neck, make sure you keep your elbows back, and crunch up using your shoulders. (This will help prevent neck soreness).
Keep your lower back pushed into the floor or bench. Unless you are advanced, and have sufficient abdominal and lower back strength, do not perform exercises that encourage you to arch your back up. If you find in general that your back arches keep your knees bent and closer to your body so that your lower back is pushing into the floor or bench.
Don't over train your abdominals. Three quality training sessions per week is ample. Remember muscle needs recovery time to grow. It's muscle growth that increases your muscle tone in your midsection. Over training will actually deplete the muscle instead of increasing it.
Don't go overboard with performing loads of repetitions. Rather than trying to achieve 5 x 100 crunches per exercise, hold for a count of 2-5, each rep you perform using 4-5 x 12 burning reps. This will get you better results. Your goal is to damage the muscle to promote muscle growth for increased muscle tone.
Don' t just train your abs. You need to be performing a full-body program rather than just concentrating on your abdominal region. Focus on training each body part so that you can a achieve a "toned and firm look" all over.