Sampattasana, also known as the plank pose is great for strengthening the entire core, from thighs and abdominal muscles all the way to shoulders. Learn and practice this yoga asana @ https://www.youtube.com/watch?v=CV_2SdPC1zc You may also refer https://www.youtube.com/watch?v=OkeXtEV_0ks for developing a complete plan for flat abs.