I always forget something. It seems like every sweat shirt I own was bought when I was on the road and shivering.
And I always pack something I don't need. I don't think I've ever actually worn that just-below-the-knee conservative black skirt.
Usually we pack the night before we leave, in about 30 minutes. For this trip, we decided to be hyper-organized about it. In large part, that's because we're determined to do this in carry-on only. That means planning on laundromats and stuff washed in sinks, the way the guidebooks nag you to do. And it means making an insane number of lists.
It also means paring books to a minimum. We're experimenting with electronics to do that -- one Kindle, one iPod Touch, both loaded with lots of reading material. We have a few little tiny pocket travel guides (plus the Lonely Planet chapters on the iPod). We're skimping on bird books, too, although the one for Bali was slim enough to bring along. Beyond that, we've agreed we can each carry one paperback that we can recycle along the way.
As of right now, it all fits in carry-on, with room to spare for stuff ... because I always forget something.