I am not sure if this would work for you, but it does for me.
IN everyting you do, when you walk, you sit, (except siguro if you eat), always try to squeeze in your stomach.
At first, parang nakakapagod kasi papaliitin mo ba naman tiyan mo, but eventually, pag nakasanayan mo na, ma rea realize mo, you are squeezing your stomach unconsiously na, all the time.
This is good not only for pampaliit ng tyan but it will improve your posture.
Hope it will work for you..=)