your abs are like any other muscle in your body, train them like you train anything else.
im currnetly doing a full bodiy workout, so i train mine 3 times a week. but even once a week would be suficaint(sp?) or posible even better.
as for exercises, cant go wrong with decline wieghted situps, and leg raises. but remember, you midsection is worked when you do any big movement, eg squats and deadlifts.
im sure this has been brought up before. SEARCH.!!