If you are wondering what is the one single best thing you can do to prepare for a long trip this is it: pretend you are leaving one week before your departure date.
But seriously, you have to convince yourself. This is like setting the clock ten minutes fast so you arrive on time. There are ALWAYS going to be things that you have left to the last minute or problems that just pop up; you will have forgotten to tell your bank you will be using your debit card overseas, you might not have to thought to make a photocopy of everything in your wallet in case it gets stolen, you may have meant to line up somebody to take care of your daughter's hamster but not have followed through yet. This way, with this one little trick, you have a whole extra week to straighten out each snafu - during business hours, calmly, with no panic.
This is especially important if you are bringing more than one person. Because, for example, if you are traveling with a 14 year old boy the packing scenario will go something like this:
6 weeks ahead. Me: "You sure you have everything you need for the trip?"
4 weeks ahead. Me: "Have you made sure you have enough clothes that still fit for the trip? You've grown about a foot recently."
2 weeks ahead. Me: "I'm taking the girls to get some shorts and bathing suits for the trip, do you need to get anything?"
Day before Departure: Son: "Mom! all of my pants are too small, the one bathing suit that still fits has a giant rip in it and I lost my sandals!"
Last year the last two days before departure were a blur of frantic packing and last minute details. Now, I have zipped every suitcase, printed out each hotel & flight reservation, spent hours on the phone successfully straightening out wire transfers and credit card upgrades. I even made sure my miles were credited to a frequent flyer program (that is a first). I even have time to take my girls to one last mani/pedi before leaving for the airport on Monday. Of course, if we had actually had to leave a week ago I'm sure we would have physically made it onto the plane. But there would have been tears and probably much more shouting.