Whereas the Queen and Prince Philip’s household was very strict, it appears from the outside that the divorce of Charles and Diana, and the Princess's untimely death, left the boys rudderless.
The armed forces provide a structure for William and Harry. This translates in self-discipline that bodes well for the future, all those occasions on which they have duties to fulful but really would rather be doing something else. Training will help get them through those days.
The other advantage of the armed forces is well, like the movie title which says it all: being An Officer and A Gentleman.
They say that psychology follows physiology – and one hopes with time that the bearing which comes of being a a serving officer in Her Majesty's Armed Forces will teach William and/or Harry that we shouldn't bear witness to a similar spectacle as that which surrounded the older Prince and the end of his relationship with Kate Middleton.
With practice, they will learn that Gentlemen do not dump their sweethearts in front of the media – they never lead them on and they always see the lady to her doorstep. (In short, listen to Boyz II Men and the track All 4 One’s I Swear).
If it is true that the Queen Mother ignored Prince Harry in favour of William, it must be due to forgetting her own husband was also a second son.
The best planning, however, is to educate each and every one in the succession line. Not just the heir, not just the spare.