Below is an STX K18 video featuring Kyle Harrison, one of the best midfielders in the game. He describes how he practices his shooting in his own time giving great emphasis in practicing at game like pace. Shoot at practice like you would shoot in a game!
One of the best attackman ever to play the game of lacrosse is Tom Marechek. He is one of the best shooters ever and has absolutely amazing stick skills. He has released a DVD called Behind Lacrosse: Shoot Like A Pro with Tom Marechek and I highly recommend this to all players AND coaches!
I think one of the most important things is that as a player, you should practice your individual skills in your own time. The best lacrosse players in the world did this from a very early age and it has unlimited rewards. If you are a coach encourage you players to do their own individual lacrosse drills in their own time.