Good advice :)
Overrating potential is a mistake i made early on in the game and i payed for it slightly but my team is gradually getting better.
Also i'd like to add something: don't trust the DMI, it is an incredibly ROUGH guide as to how good a player is, it also is greatly affected by the player's game shape. Generally especially at the beginning of a season when salaries have been updated its a good idea to check that as well. Like Aussie dude said the younger the player the better the train-ability but other factors such as Height come in to play.
Obviously if you have an 18yo with P.Allstar potential, if his salary is like 1k it means his skills are rather low. You want your trainee to have at least decent all round stats when you start so be mindful of that.
hopefully you get what i mean, its been a long day