I wouldn't think that supplements would help you attain a flatter stomach. It's strictly your body fat %.
Do some more cardio, eat right, keep working out your core and stretch. I got a flat stomach doing that.
E92 335i: Space Grey|Coral Red|Aluminum Trim
Hold out for the BRZ STI or... Rocket Bunny FRS. Update: Scratch that. I've got a 2015 5.0/6MT with my name on it.