All the sit-ups, crunches, & planks are great BUT, (no pun intended here) when you're looking for those "flat abs" you gotta watch your diet too. In most cases, with fat loss comes butt loss.. wamp, wamp! SO, to conquer the butt loss, you gotta do weights. Adding a consistent weight routine will give your derrire that full butt appearance you're not wanting to lose and give you the flat abs you're wanting to gain.
Many sit-ups per day. Start at like 15 or 20, then escalate as develop the stomach muscles. Another tip is to also burn off calories by doing cardio excercises and strenght excercises that don't target your butt.