I think a simple rule all new players should observe is salary at least $3,500, potential all star and age 21. Never ever train anyone with worse than those characteristics.
I agree with most part of this but not the all star potential bit. New managers don't need to have players with high potential. They simply don't know enough about the game to train a player with high potential (and most likely they won't be capped because new managers are usually in the low divisions, thus limited net income).
But really true, new managers waste too much money on some silly things, and I was one of them :P and I'm pretty sure many of the experts made silly mistakes at the beginning.