You don't. You just procrastinate till the very last minute before you leave, and hope you have everything.
As I am packing for Montana and other bananzas, I have realized some "to do's" and "not to do's."
1. Do not pack the pants you want to wear on the train at the bottom of your already packed bag.
2. Do pack your underwear, and not leave them in your drawer. I mean, I've never done a thing like that before. I just hear it's embarrassing when someone just opens a drawer and finds somebody's underwear. I mean I wasn't embarrassed when some other people found my underwear. I mean, not that it's ever happened to me before. It happened to my friend once.
3. Do not pack things you don't need, like for instance a dictionary. (i don't know how that crept in my bag. seriously.)
4. Do not eat food while packing. Especially things with crumbs. I think my jackets and sweaters may have the aroma of Salsa Sun chips forever, or at least until I wash them.
5.Do pack some laundry detergent so you can wash your Sun chipped clothes when you get to Montana.
6. Do not try to weigh your bags on a scale used for human beings. It's not entirely accurate, especially when the scale is consumed by your crap. Your bags don't like to flaunt their weight to the whole world! They're shy.
Yep. Better start packing. seriously.
