Making paper planes is a fun pastime that started as a type of origami and quickly embedded itself into mainstream culture. There are thousands of designs for fashioning paper planes, but there are few that fly for more than 10 seconds. The key is to understand some basic principles of aeronautics--the same principles that keep regular airplanes flying for hours can be modified to keep your paper airplane in the air for a long time.
Ensure that your wings are dihedral. Dihedral wings fold upward, not downward. Look at your paper plane from the front or back; if the wings are pointing down to the ground, readjust them so that they point upward.
Make sure the plane is stable. Stability in aeronautics doesn't mean that the plane will come apart; stability means that the plane has a center of gravity at a neutral point where the plane will return to if disturbed. A nose-heavy or back-heavy paper plane will most likely fly for only a few seconds. For the common dartlike paper airplane, the plane is stable if the neutral point is half the distance from the nose to the tail. Balance the plane on a finger or small object. If it doesn't balance at that central point, readjust the weight by refolding the wings or nose.
Throw your plane as high as you can to maximize time in the air.
Throw the plane as fast as possible. Ken Blackburn, the Guinness world record holder for the longest paper plane flight of 27.6 seconds (as of June 2010), estimates the plane leaves his hand at 60 mph.
If you have a pointed-nose-type paper airplane, add a couple of layers of tape to the nose or add on a paper clip. This adds a little weight to the nose and improves stability.