There is insufficient kinetic energy in most light ac at cruising speed to convert into the potential energy of altitude - 'converting speed to height'. The best solution is to fly the ac in the most energy-conserving manner until the IAS reduces to the normal gliding IAS - i.e. don't manoeuvre energetically to achieve gliding speed and avoid causing any buffet. It's simplest just to maintain cruising attitude and then to adjust gently to the attitude for best gliding IAS as the speed bleeds off.