Rod Tip Control = Fly Line Control
One simple rule of fly casting is that where the rod tip goes the fly line follows. Seems blaringly apparent right? But it really helps when you understand this principle and practice with it in mind. Good casters throw tight efficient loops by making the rod tip travel back and forward in a straight line. Consequently, the line travels back and forward in a straight line.
Several of the most common errors in casting disrupt the straight rod tip path. These can be but are not limited to over or under powering the cast for the amount of line, tracking errors that cause the rod tip to deviate side ways, not applying power to the cast smoothly causing shock waves, or arching the rod tip path by allowing too much wrist rotation.
Practice making the rod tip travel in a straight line on the back and forward casts and you should be happy with the results at the end of the line.