when you go to the pool look at the difference between great swimmers, bad swimmers and good swimmers. It looks easy if your doing it right, just like running. Be long and in the water with short kicks.
The worst offenses I notice is short choppy strokes, no hip rotation, and kicking with bent knees and waist. They are all related.
One more thing there has been some recommendations about "gliding in the water". Don't be fooled you never stop and glide in the water. A good swimmer taking long strokes, rotating hips, and kicking properly appears to be gliding in the water but is not gliding. A good swimmer is always advancing thier stroke.