One trick I always used but never understood why it worked:

When you add a crease to the paper airplane, (say, folding a square corner in on itself at the nose or the tail), it weighs down that part of the plane. So if your plane is pointing up too much, you add a crease in the nose and it flies more level.

But why should adding a crease increase the weight of one section of a plane? You're not adding any more paper.