Also YouTube stomach vacuums, you can do them all day wherever you are (kinda like kegels) and their simple and work the most forgotten part of the abdominal system.
Why abs* on a skinny guy **
You get a flat stomach by dieting, any exercise will help build some muscle there but most people have a six pack underneath the fat already, and everyone has a "flat stomach underneath the fat.. This is why and on skinny guys don't count :P
Also, a lot of people's pooch or whatever you call it has a lot to do with inflammation of the digestive track. Eat cleaner foods, try cutting out gluten and/or dairy, as those seek to be the most common sources of inflammation for most people.
Or try going a week without any carbs and I'll bet your stomach starts flattening out pretty quickly.
Of course crunches will help but it's good to work other exercises like leg lifts and all the variations of leg lifts into your routine. Crunches and leg lifts alone won't get that flat stomach you seek however. Nutrition (diet) is the most important component to working towards a flat stomach. Good luck!!!