By trying and failing, the players can learn what doesn't work. By trying and succeeding, the players can learn what does work. In both cases, learning has taken place and the players have the opportunity to improve. Variety, Competition, and Fun. When interest is high, the players learning rate and capacity are increased.
Variety can include adding new or modified drills periodically.
Competition can be added to any existing drill where two or more players execute the drill simultaneously. Your players will tell you what they think is a fun part of your practice and you can decide when or if to incorporate it. This may be a scrimmage or a favorite couple of drills. Listen to your players and use this information when planning the fun part of your practice. If you incorporate these factors into each practice, you will be amazed at the work effort from your players.