Inspired by the thread Parseltongue Phoenix made; it's pretty simple:
What if Dumbledore told Harry about the Prophecy at the end of 4th year instead of 5th year?

The attack by Dementors in 5th Book and the chance of being expelled from Hogwarts in the trial could provide him with proper time to actually train, or begin the Hocrux hunt a year early with Dumbledore getting expelled by Umbridge for whatever reason she cooks up.

 

I really do mean no offense by this, but the other idea simply is better than this.

See, this one doesn't actually affect much to Canon!Harry - hell, Dumbledore could've told Harry about the Prophecy in the end of book 1, and he still wouldn't have changed to train more. The only way that this would start a basis for a new, y'know, CHANGING point, is if the writer decided to change Harry to be less of an idiot. And that right there is the flaw - there are better instances and ways to change Harry than just having a "Dumbledore tells you to do shit" copout.

 

I really don't understand why everyone keeps insisting that canon!Harry should have started training more. And I really don't understand why that'd be a measure of his cleverness. Canon!Harry is plenty good. JKR left enough indicators of that. What she did do was have Hermione doing all the magic leading to the belief that canon!Harry is an idiot.

Really, are you suggesting that by training for seven years, Harry'd defeat Voldemort in a duel? Dumbledore would not have defeated Voldemort in a duel at seventeen, heck he couldn't do it at a hundred and fifty. If Harry defeated Voldemort in a duel, it'd change the whole thing into a book parallel of shonen manga, and it's enough of a travesty as it is. Which is why Harry did the smartest thing to do at that point which was to rely on Dumbledore's wisdom.

Back on topic, agree with Oz. Really, the only thing that'd change would be Sirius not dying the way he did. But unless Harry was good at Occlumency, the author'll be pressed to get a proper reason for Dumbledore telling him the prophecy.

 

It's my believe that even a crappy REAL fight is better than unexplainable deus ex machina. And you can make Harry do well so that he's just more competant, not the greatest wizard that ever lived Super!Harry.