In order to flatten your stomach and reveal your six pack abs use these flat ab exercises for great effect.
Doing regular ab crunches are not the way to go.
They strain your back and neck.
Instead use these.
Crunches on an Exercise - By doing crunches on an exercise ball you will really target your abs and eliminate the strain regular crunches give you.
How to do them:

• Lay on an exercise ball so your back is centered on top.
  
• Then arch all the way back to take advantage of the range you have.
  
• Then move forward for the crunch.
  

Tip: If you want to make it more challenging simply hold a weight above your chest.
Hanging Knee Raises - These are great for your grip strength since you will be hanging.
They are great for targeting your lower abs for a fuller six pack.
How to do them:

• Grab on to a pull up bar.
  
• Keep your body straight.
  
• Without swinging simply lift your knees up to just about touch your chest.
  
• Then bring them back down to the starting position.
  

These are some effective flat ab exercises but to be honest they are not that effective for burning your belly fat.
If you want to get rid of your belly fat to actually see your six pack abs then you will need to focus on three things.
They are diet, cardio, and compound exercises.
Each workout should mainly focus on compound exercises like squats and deadlifts since these are the real fat burners especially for around the belly.