Explanation of player positions and what type of training they should receive, this is roughly what seems to work the best for each position, again this is only what seems to work best for me, ultimately it is up to you how you train your players.
Best skills to train by positions:
Center Forward = Shooting, Speed, Head Shot or Dribbling
Inside Forward = Shooting, Speed, Head Shot or Dribbling
Hole = Head Shot, Shot, Long shot or Dribbling
Offensive Midfield = Long shot or Passing
Center Midfield = Passing
Defensive Midfield = Tackling or Passing
Right Wing and Left Wing = Shot, Speed or Dribbling
Side Right Midfield and Side Left Midfield = Speed or Dribbling
Center Back = Tackling
Sweeper = Tackling
Full Right Back and Full Left Back = Tackling
Wing Right Back and Wing Left Back = Speed or Tackling.
Goalkeeper = Goalkeeping
As well as the skills mention above there is the option to train stamina, i believe all this really does is maintain a players energy level for longer.
For example 2 players both start with 100 energy however one of them has stamina skill at 10 and the other has stamina skill at 20. The player with the higher skill in stamina means his energy will last longer, so again for example come half time the player with 10 stamina skill may go down to only 50 energy left where as the player with 20 stamina skill will maybe have 60 energy left. I am not sure about the precise math but if anyone wishes to share that with me i will add it to the post.
Speed also seems to be a key skill to train and for the most part seems to boost every position, not something i am concentrating on at the moment apart from the obvious pointed out above, but hopefully some more details to follow soon.