Definitely this! My parents wound up missing their vacation last year because they didn't look for their passports until the night before and weren't able to find them in time to make their flight. Had they looked earlier, they could have paid for rush replacements.
I also go over reservations and plans with my husband and make sure he is happy with everything. He basically doesn't pay attention to vacation planning at all until right before, so it's my only chance to get his input. Then, I print out confirmations for everything. I also confirm dog-sitting arrangements, stop the mail and put in book requests through the library if there's anything special I want to read on my trip. (Sometimes, I like to theme my reading to the trip).