Lucas said it himself when he wrote a letter to the producers of Lost:
“Don’t tell anyone … but when ‘Star Wars’ first came out, I didn’t know where it was going either. The trick is to pretend you’ve planned the whole thing out in advance. Throw in some father issues and references to other stories – let’s call them homages – and you’ve got a series.”
Source? (Not that I doubt it, but it’d be nice to have the proof for these very kinds of dscussions)
I read it in the L.A. Times, but it was carried by lots of reputable publications when he originally sent it.