Katia:
when you get Ab Hits, or when you alternate between doing the varying ab workouts you have already in your collection, try to make sure that you alternate between the type of exercise that you do for the abdominal muscles so that you hit each and every musle eah week in a variety of ways. This will ensure that you strengthen the entire core/abdominal region, not just keep hitting the muscles that you favour from the exercises that you favour.
I like the ab routine from MIS, it's traditional abs and very thorough. I use this as my base and do this once or twice per week. On top of this I add my favourite ab routines which work more the core and these moves come from Cathe's newer routines. I personally like the Ab /core premix from Bootcamp a lot, also the core work from Cardio & Weights and also from PUB. These are my staples to keep the whole region in shape.
The extent to which your stomach "looks" flat however, regardless of how toned the underlying muscles are, is a question of genetics and your predisposition to keep a little fat there. Maybe diet and cardio work can help with this, but genetics is a hard nut to crack, so some of us have to accept that we will carry a little more fat there than Cathe, for example, and just get on with our lives!
Clare