This may be very disappointing to some. But it is the truth. If you want nice abs you are aiming for an entire body transformation. Period.
How do I get abs of steel?
Have you ever seen anyone with well-defined abs who also had fat thighs, "man boobs" or flabby hips? I can bet you haven't. That's because it's near impossible to achieve localize fat loss. The body tends to burn fat from all over the place, and generally you have no control over which body part will slim down first. NO SPOT REDUCING
Lower Overall Body Fat
To see the abs you've got to burn the fat first. Generally you have to get your whole body to <= 10% body fat (men) or <=14% body fat (women) before you start seeing any definition. Coupled with the regular resistance training to build and define the abdominal muscles.
Train the Abdominals
To burn more fat you need to increase cardiovascular exercise. Abs can be trained like every other muscle (e.g. twice a week). Also note that our muscular structure is individual depending on genetics - that is why some can only get a '4-pack', while others show off an '8-pack'.
Should I do hundreds of crunches?
Any muscle definition in the body comes from the right nutrition, and the right mix of cardio and weight training. Repetive abdominal exercise will give you great endurance, but probably won't give you that washboard you're wanting.
Ab Exercises
There are 3 main abdominal regions that should be worked:
Upper Abs
Basic crunch, Swiss ball crunch, Decline crunch, Bench crunch, Cable crunch
Lower Abs
Reverse crunch, Hip up, Reverse Decline crunch, Hanging Leg Raises(tend to recruit more so hip flexors, than the abs), Swiss ball leg lift
Obliques
Side crunch, Cross crunch, Swiss ball side crunch, Decline twist, Twisting Cable crunch.
Tip: When working the abs, suck your stomach in (imagine pulling your bellybutton towards your back) and concentrate on feeling your stomach muscles being worked. It is very easy to use bad form and end up working the hip flexors. | 
Everything begins with a dream! 
