How long does it take you to pack a backpack or Camelbak for a hike? How about a overnight backpacking trip? How about just a simple business trip or overnight stay at a friend’s house? An hour? Two or more? How often do you forget an essential item that you have to go without in the woods or buy a replacement for in civilization?
Avoiding any unnecessary headaches with packing and organizing all your crap for a trip can be quite easy. Just pack all of it before you even know about your trip. There are definitely some items that will be adjusted to each situation, but the vast majority will always be there. You may need a stuffable down jacket in the winter, but that knife, flashlight, rain shell, etc. that you take on every trip can be put into a bag and left there. Stick that bag in your closet or car, and pull it out the next time you go on a hike. If you have enough duplicates (I know pretty much everyone here does), make a few bags for trips that you go on several times a year.
If you do not have duplicates, then still go through the exercise of packing the bag. Once you are done, unpack it and make a list of everything that is in your pack. Keep this list and pack by it the next time you are in a frenzy to get ready to go out of town. Even if you have separate bags packed, it is still a good idea to make the list.
When you get back from the trip, unpack your bag again and take a look at the list. What did you use? What did you not use? If you used something several times, it might be a good idea to take a replacement along with the original. If it is something your life depends on, definitely take a replacement. If it is something you never use and can live without, leave it at home. Do you really need five pairs of underwear for a week in the woods? Is a plastic poncho going to be sufficient for backpacking in Olympic National Park? Should you really leave that Epi-pen at home even though you have never used it? Every time you get back from a trip, repeat the process of refining your list.
If you already keep lists, share them with us over in the wiki. There is an area for recommended gear with some ideas for lists. Feel free to create new ones if you have one that does not apply. If you have one that is radically different than one that is already there, please create a new one instead of editing the old one.
If you have any ideas on how to get ready more quickly, please share.