It's in the details

So, I'm re-reading Deathly Hallows in preparation for the movie this Friday.  I do this every time -- cram the book in one last time so it's fresh in my memory.  That way when they screw things up in the movie, I can still explain to my sister what's going on. (She hasn't read the books. She's one of those Twilight lovers -- and I try to explain how much more amazing HP is, but she doesn't listen.)

Anyway, back on topic.

Reading again, right.  Okay, so as we said in the comments yesterday -- reading this series, any of the books really, leaves me in awe of the amount of planning JKR did.  The way she wove those books together was masterful.

What I'm trying to grasp from this reading is how to effectively plan a series.  Did she sit down and plot out all seven books at the onset?  My gut says no.  She could have, but I don't know.

Either way, how do you handle such a massive undertaking?  Did she sit down to write the second book and go, "Hold on!  Let me go back to book one and see what I can pull in here to make the story richer."

How many of you have written, or are writing, series books?  Two, three, more?  What do you do to make sure details are carried from one book to the next?  Do you intentionally leave clues in the first book to feed the second, or do ideas stem from what you wrote and blossom in the later editions?

My current WIP is heading the way of two or three books, and though they've been in the back of my mind stewing on plot and characterization, I haven't sat down and really planned anything.  I've got a few  notes scribbled here and there, but that's all.

Tell me, lovelies, what do you do?  Or what would you do if you found your WIP couldn't be told in just one book?