Decide on your Daily Minimum Workout
 

Be the best you can be at your sport.  Work on your skills year round.  For example for Lacrosse you should be playing wall ball year round to keep getting better!
 

Plan on working out year round in HS.  If you are a 1 or 2 season athlete, then consider adding cross country or track & field in your off seasons.  It will not only keep you in shape, it will also keep you on the same schedule all year round. 
 

Here is a simple year round workout plan to use when you are not in season.  If available you can work in weight training etc.  Lacrosse specific workout plan.
 

Know when it is time to take a break. Over extending yourself will lead to injury.  One day a week is always great for complete rest.  
 

For example if you are running cross country six days a week but your main sport is Lacrosse and you play on Sundays, then be up front with the CC coach that you need one day very easy or completely off!
 

Make sure to incorporate regular body weight workouts year round at a minimum.  Squats, push ups, planks/core etc.
 

Be coachable!  Be on time!  When practice starts you should already be on the field throwing a ball around etc.

Always ask your coaches how you can get better.  How you could fix what caused a mistake in a game etc.
 

Play several sports in your HS besides your main sport. 

Enjoy the experience, the fun, the memories and teammates. Like we said before it will keep you in shape for your main season.