I really don't do much ab work. I do TONS of cardio & lots of strength training. But I am always very conscious of my ABS. Even when running on the treadmill or cycling on the bike or doing any strength training I always pull my ab muscles in very tight. I think that is better than any crunch u can do. But I am by no means an expert. ;)
Unfortunately you can't spot reduce fat. Our bodies don't work like that. Just eat a clean diet full of veggies and fruits and lean protein. Diet is the best way to lose fat. The gym is where you tone.
Never anything wrong with working out your abs even if you can't see them. In fact, work them out often! Because when you start eating clean and doing intense cardio and the fat begins to shed, you will see the muscles you worked so hard on ;)
Keep your abs tight with every excersize you do: those muscles are your core strength and without a strong middle you can't do much. Be patient, fast doesn't just fall off and 6 packs don't appear out of thin air. It takes time, hard work and dedication but you will do it :)