How would you redesign your paper airplane to make it fly faster?

You can get a plane to fly faster (for a given force applied) by decreasing drag, e.g. reducing the width of the fuselage or wings (the latter also reduces lift).

If you add weight to a paper plane, especially at the nose, you can get it to fly faster for a given acceleration (i.e. the throwing motion) -- the additional weight will increase the speed gained as gravity drags the plane down, because the air resistance is a function of the area, not the weight. A metal plane will fall faster (and with a higher terminal velocity) than a paper one if they have the same resistance area.