Summarize:

The fuselage is the bottom part of the plane. In most paper airplanes, this is where the fold separates the two sides. You've worked to balance your plane, so grip the center of the fuselage with your fingertips. This is where the plane gets a lot of its stability. More delicate planes are better gliders. A rough launch will damage them and ruin their flight trajectory. Bring your wrist forward in a pushing motion. Keep the plane level with the ground. Planes with short wings function better with a hard launch. Angle your plane upwards. Use the same pushing motion with your hand, but apply more force. If the plane is a dart, it will stabilize as it descends. Non-dart glider planes should be throw upwards gently with a pushing motion.
Hold the fuselage. Throw thin, long-winged planes gently. Throw short, stout planes upwards.