When first learning to play, many drummers find the heels-up method easier and often this is the only method they learn.
  They simply stick with what they know.
  However, by taking this approach they are selling themselves short and limiting their ability to improve quickly.
  Each approach uses many of the same muscles and by "cross-training" these muscle groups you can increase both speed and control when playing.
When you are playing double pedals you want to keep both pedals balanced and even so that when you are playing them together you don't unintentionally sound heavier on one side.
  To do this, find the sweet spot (or balance point) on both pedals that will let you do the least amount of work.
  Use the same point for both pedals.
Another important technique that can be played on any pedal (single or double bass) to produce fast double strokes with your feet is the heel-toe technique.
  Just like using a double stroke roll on a snare drum, the heel-toe technique allows you to play doubles easily with more speed and precision than can be achieved otherwise.
When you play this technique your heel hits the pedal first making the first beat, then your toe hits a second beat right after using somewhat of a rocking motion.
When you use this method, your foot needs to be high enough on the pedal so that your heel can push the pedal down.
  It is not as easy as a normal hit is, but when you get the rocking motion down you will find that it does not rely on pressure only - you also get the benefit of the natural pedal bounce.
  If you have a toe stop you may have to move it away as far as possible or remove it all together.
  It is easier to play properly barefooted since shoes reduce your feel with the pedals.