When flying commercial airliners such as B757's and A320's I cannot raise the speed of the plane past 350 knots. Can someone tell me how to raise the speed of the airplane without getting 'OVERSPEED" cautions?Thanks

At what altitude are you trying to raise the airspeed above 350KIAS? If you are at FL300+, going 350KIAS is too fast for most commercial airliners, you need to use Mach at those altitudes.