By far the most important factor in the success of any off-season training regiment is accountability. In my opinion, even the most well-thought out individual workout plan isn't going to be any good if you cannot verify that the player actually followed the plan. We've all heard of players saying they are going to work on their game all summer, only to find out they scarfed down Doritos all day and played PS3 with their buddies all summer long. Professionals do it too, Shaq always claims he's going to maintain his playing weight through the long summer only to come into training camp well over 300 pounds. It's human nature, people slack off in their days off and unless there is some kind of record keeping and accountability, it's nearly impossible for most people to stay on track otherwise.
So, how exactly do you accomplish it? I'm no expert but here are some of my thoughts.
Rewarding Attendance
Because accountability in staying with a training plan is arguably more important than the individual results, the incentives should ideally be structured in such a way that you reward attendance as much if not more so than the results. In this way, its slightly inverted than during the basketball season, where you want to reward performance, and not simply showing up. It matters less what your 40-yard dash time is, then it does how many days in a month you actually showed up to workout and the progress from week to week.
If you keep the attendance records or daily logs online or on a big board in the weight room, it serves as its own positive reinforcement, as social pressure between teammates helping to motivate each other and to keep each other honest.
Buddy System
Pair up individuals so that they have a training buddy. Working out in pairs helps keep both individuals on schedule and on task. Will it work all the time? Probably not, there will be times when both individuals slack off together, but it certainly helps to have that other person nagging them to work out.
Additionally, working out in pairs is safer and more effective. For example, nobody should be running alone in the park especially with iPods all the rage nowadays and all weightlifting should be done in pairs anyways. Working out in pairs is more fun as well as there are more activities you can do with 2 people that you can't do alone, especially basketball-wise.
Daily Log
The beauty of the Internet is that you can have players use an online dairy to post important information like daily heartbeat, lifting numbers, body weight, and even daily dietary intake. As a coach, you can login to track their daily progress. Now, of course, some players can and will cheat by not doing the workouts and just entering in phony information. But the reality is, if someone is so lazy as to not bother to work out all summer, they're probably even lazier than to go in and enter fake numbers every single day. This is also pretty easy to verify, because if players are paired up in the buddy system then its easy to know if one or both have skipped workouts and just entered phony info. Also, if they're entering in numbers every day and they show up 30 pounds heavier for the first day of school or training camp, it's obvious that they've faked it.
In terms of what specific individual skill development fundamentals players should be working on, I don't think there is necessarily one size fits all kind of system. Ideally, each player should have a program that specifically addresses the areas in which they need to improve on. On a more general level, players should be working on common fundamentals such as shooting, strength, and endurance. For more ideas on workout specifics, check out Alan Stein's Pro Power 2-Pack DVD which talks a lot about basketball specific strength training.
