Buy longer drapes. To make the window look taller, you need longer drapes or curtains. Measure from the ceiling to the floor and buy drapes that length.
Raise the curtain rods. Instead of having your drapes mounted just above the window, go to the ceiling. Mount the rods as high as possible for the length of drapes you bought. You want the drapes to just brush the floor.
Add a valance. This will also trick the eye into making the window seem taller by filling in the space between the rod and the window.