Best way to get a flatter tummy?

ok ..well I am 5'5" 125 pounds. and I am definately not fat but I did have a "squishy tummy" as you called it from having two kids... (obviously I'm older than 16.... Actually I'm twice that LOL) but anyway my mom gave me this exercise ( she is an O.T.) it worked for me. so what you do is lay flat on your back with your hands palm down on the floor next to you... lift your legs straight up ( like a 90 degree angle) then slowly lower them until they are about 3 inches from the ground and hold that position to a count of 5 ) relax and then do it again.. ( I did mine 10 a day for like 2 weeks and I lost one pants size but no weight.) ( its not as easy as it sounds trust me.. it makes the muscles in your stomach work in ways other exercises dont