I'm sorry I must disagree. J.K. Rowling was able to come up with a whole new world from the top of her head. I know many people find older books to compare the Harry Potter series to, but reality is: it was HER idea. She came up with characters, made them like you and I, she made hundreds ! Check wikipedia, a list of all th Harry Potter characters, and you'll see how many there really are. You can't say she found all of those at the back of a book. And I know that's not what you're saying, but some people do. Also, some things may not make sense at first, but they do all add up. You're talking about potions. Lets take an example from the most recent movie; Harry Potter and the Half-Blood Prince. Harry wins a Potion called Felix Felicis. Liquid luck. Something everyone wishes for. Rowling took ideas that many people wished for, and transfered it on to paper. This liquid luck potion came in handy for Harry later on. I think she truly had a plan, all the way through. Of course she added twists and turns, that's what makes a great story, great !
Spoilers :
In book one, Harry learns the base of his story. His parents died from the attack of the most dangerous Dark wizard, his mother died to save him, and he came face to face with the killer. He learns about his followers, the magic, his name. He learns that love can truly save someone.
In book two, Harry learns that Voldemort and himself have a connection, they can both talk to snakes. He figures out who is behind the opening of the Chamber of Secrets, Tom Riddle. Lord Voldemort. He learns more history about his foe, meats new people, like Dobby. Hence, learning about house-elves, how they are freed, etc.
In book three, Harry sees that someones closest friend can be unloyal. Harry's parents were betrayed. As the series was continued to a third book, he started to learn more about the history of his parents death. Then he meets his godfather, one adult that he can truly connect with, and possibly live with in the future. Professor Trelawney is introduced, a very important character come later books. He also meets Lupin, and learns about Fenrir Greyback.
In book four, now this one's a big one. A big tournament at Hogwarts, Harry is too young to compete in. But somehow, he was chosen. There were only supposed to be 3 but he was the 4th. Someone was planning something. Trying to get Harry killed. At the end of the novel, we learn that Voldemort has returned, in his own, proper body. Someone is killed, and once again, we see a connection between Harry and Voldemort (their two wands had a connection).
In book five, Harry is deceived by the Ministry. They are turned against him, no one wants to believe the truth. But there is a group of people that do believe him. The Order of the Phoenix. They are out to fight the Dark Lord, and they stay by Harry's side. Harry ends up losing someone dear to him, his godfather. J.K. Rowling had planned for another death, but could not bare to kill the other one off. This shows that she had a plan, it wasn't at random.
In book six, Harry goes way deaper in Voldemorts history, he learns about Horcruxes, the Dark Lord's family, his greed, his power. Once again, another death arrives, you know who it is. It was a terrible loss. One of the worse. Not only was it sad, but it made it more difficult for Harry to pursue his mission. We learn about the importance of his friends as well.
Finally, book seven, all is reveled, the end of the story. Although we find an all new plot, the 3 deathly hallows, it is obvious that it was in mind for her to do this since book one. Dumbledore had the cloak to make sure that it was truly the real one. I just don't think that she came up with this "on the spot"
I'm sorry I went on a rant, but this proves that she had a plan. She could have put it all in one, or three, but that might have made the story extremely hard to follow, and very uninteresting. Harry is a teenage, and it wasn't ALWAYS about Voldemort, though it was often, not always. He has hormones like many, he has enemies like many (Malfoy), he's a regular boy, but yet, he is not. I can't explain how much I idolize Rowling for her books. I know that a lot of people think it's nerdy, or dumb. And I think that's their opinion, no one HAS to like something. The only things I don't like, is when someone shoots me down for liking them (because I don't just like it because of the "magic", there's so much more, friendship, family, love, even the writing, i love the writing, her plan. Even if I just liked the fiction, who cares), and when people think that it was just a fluke that Rowling thought of this story. Once again, sorry I went on and on, but I hope I made my point clear :)