Again like BJS said, you'll get a ton of responses on this but here is what I do.
Best fielding instructor I can get.
18, 19 and 20 year olds start in low A, 21 and 22 year olds start in High A. They get promoted at the end of every season. Have them pitch or play as much as possible (I have 20 medical and training so I ignore their health. They can play until their arms fall off for all I care). I won't put bullpen guys as closers just because if the team sucks they won't pitch as much. Invite them to spring training, make sure batters get at least 25 plate appearances and pitchers get into at least 5 games, although I usually try to have them play as much as possible.
* A bit of reasoning behind why I do this.
Guys get drafted and then the rookie league doesn't start for about a week. Considering that players gain ratings based off of playing time I want them in games as soon as possible. Throwing them into Low or high A means they start playing the very next day. In theory your best prospects should be good enough to play at those levels.
Guys usually take big leaps when they play in spring training. 2-3 points vs usually 1 point at other times in the season. Its important to get your prospects in as much as possible.
Promoting at the end of the season ensures they don't leave. I usually end up with a bottleneck at AAA but I really don't mind.