Speed and a knack for finding holes in the defense are great skills for a running back to have. But, if a half or full back can't block, he'll probably find that his agility is going to waste while he stands on the sideline.
High school coaches may be fortunate enough to find a good blocking running back on their team, but most younger players who are just learning the game usually block like they're playing in the fourth quarter of the NFL Pro Bowl. Because of this, blocking should be one of the first things taught to all youth football players when they start practicing this summer.
For the sake of learning the mechanics of good blocking, you can line up all players as though they are linemen, and just make sure they are matched up with a teammate that's of relatively similar stature. It may be a good idea to use helmet scrimmage caps to make it easier for the athletes to identify offensive and defensive players.
The first thing they should learn is the difference between run and pass blocking. Since the latter is simpler, it might be best to teach that first. During a pass block, players need become an immovable force. They of course shouldn't cement their feet down. Rather, their steps should be helping them drive their body into the opponent they are blocking.
During a running play, however, defenders will be heading toward offensive players with more momentum since there are so many more moving bodies than there are in a pass play. When blocking in a run play, backs should always keep their feet moving and their hands up. They should hit the defenders they are trying to block in their shoulder pads to deflect their momentum in a different direction.