First, make sure you are kicking up - using the hamstrings and gluteal muscles - as well as down. Then, make sure you aren't kicking too much, as a really wide splayed-out kick creates a lot of drag. Finally, make sure you are kicking from the hip, with the rest of your leg loose and relaxed.
If your balance is good you really don't have to do much kicking at all. Using the kick to bring the hips up burns up a lot of energy and doesn't really do much for forward momentum. Work a lot on the Superman glide from a wall or standing pushoff. Focus on getting a lot of distance out of each glide, with just a little light kicking.