Here is what my two boys did in the off-season in order to get ready for this year's lacrosse season.
They went to a lacrosse summer camp as well as a few fall/winter lacrosse clinics (which are held once per week and last for a couple of months).
My boys also participated in a couple of off-season sports (i.e. basketball and track). This has boosted their endurance significantly. Endurance/stamina is a vital "skill" needed for lacrosse.
Moreover, we practice once or twice on the weekends with 30 minutes of wall ball. Wall ball is a critical off-season training tool for beginners. For wall ball ideas, you should visit our Wall Ball page.
I also conduct my own lacrosse drills for the kids on the weekends... but I am nuts. :) For drill ideas, please visit our Lacrosse Drills page.
That in mind, I have created an Off-Season Lacrosse Training page that gives families a number of simple ideas in order to prep their son(s) in the off-season.
Hope this helps.