Actually just looking for advice - Was there anything in particular that you as a parent began to do in the 12-18 months prior to high school?
WestCoastPapa- while not reading every line of the responses so far, it sounds like you are getting a lot of advice from the conditioning and training aspect. One thing that I suggest you also do is check to see if the high school coach has any camps that your son might attend. Usually they are very affordable and its a good way to get on the radar early.
In our area the HS coach has a camp each winter and while it is certainly instructional, I always got the feeling that it was somewhat of an informal workout for up and coming middle schoolers to be evaluated by the coaching staff. By the time our son was eligible for high school tryouts he was well know by the coaching staff and just as importantly, jerseyson had a pretty good idea about what the coaches were looking for.