The value of back up cannot be overemphasized. If no one is there to accept a throw, whoever has the ball must run and occupy that position. Guarding the lead base is the first priority. The next priority is to stay out of the runner's way. Anyone who blocks the progress of a base runner while not in possession of the ball and not in the act of fielding the ball will be called for obstruction.
Some coaches teach their players to throw and run to the inside of the base path.
Others prefer that their defense remain to the right side, since most infielders are right-handed. Either way, criss-crossing the base path is asking for trouble. If the runner tries to obscure your fielders' throws or vision, they should move further to the chosen side. The ball handler sprints toward the runner forcing him to abandon his shuffle steps, square his shoulders, and retreat at full speed. Speed is crucial.