Everybody has heard of 10 and 2 o'clock as they relate to fly casting. Just as important, however, are the clock positions of 12 and 6 o'clock.
Picture yourself from immediately above standing on a clock face and facing 12 as you cast. A perfect cast from this aerial view will have the rod tip traveling forward and back in a straight path and stopping at 12 and 6. This is also referred to as tracking. Good and efficient casters exhibit good tracking. A common thing to avoid is casting around your body which for a right hander would look like 11 and 7 on the clock. The 7 o'clock back cast is a very common error, especially as casters try to cast long distances.
A good way to practice is to videotape yourself casting from exactly in front or behind and make sure that you rod tip travels forward and back in a straight line.
Concentrate on this aspect the next time you cast. Casting with perfect tracking will improve your efficiency, accuracy, and distance.