Planning? What's that? =P
I understand everyone writes their stories differently. It's interesting to discover, to be honest.
I often dive into my stories with nothing more than a vague plot line and character development. I have aspergers and with it comes a rather short attention span, and I just can't write a story knowing exactly what is going to happen and where, when, how. It would be like reading a book for the second time. Something I can very rarely do. I like to be surprised as I write, and discovering myself how everything pieces together. It works quite well for me. The trick behind this is I do NOT submit new chapters until I've written enough to be satisfied I won't change anything. Sometimes I go back to tweak things to make it flow more smoothly. (For example, currently Glitched has 9 chapters submitted including the prologue. But I've written 21.) My mind is always working planning my stories for me and getting me all excited to write more. The days where I have writer's block are rather blue for me.
And since I'm working on three novels atm, two of which aren't fanfiction, that's a real pain...
I can't write a story with nothing to go off though. (That's how I end up with 'crack fiction' XD Something else I love to write lol) With more serious novels, I do some amount of world building and character development before hand, and note down major plot elements, but that's the extent of my planning. (The world building I did for Glitched means it's had a lot more planning than The End where I did virtually none! I think I did nothing more than just plan out the characters there... I don't think I've ever been more excited about writing either. If I wasn't sat typing at a computer at the time, I'd be waffling away to my husband about everything that popped into my mind from character names to plot direction XD)
I get more ideas while I'm walking, listening to music, daydreaming while doing the dishes, then write them in. It helps keep me excited about my writing when I discover something new and can't wait to explore it more! And when you're typing away and an idea crops up and surprises you, throwing the story in a different direction as the characters almost seem to make their own decisions, that's one of the best things about writing imo!