Packing for a Long Trip

I pack last-minute.  Always.  And I almost always under pack.  So how do I pack for a long (in this case, 3 weeks) trip?  It goes something like this:

Step 1: Throw everything including your smallest suitcase on the very messy bed (you're going to take the sheets off and wash before you leave anyway).

Step 2: Sort through and get rid of stuff from the last trip that you know you don't need.

Step 3: Consult your randomly-scribbled list and throw some more things on the bed...go in search of the things you have no idea what you did with after the last long trip...freak out because you can't find that one thing you absolutely need, add some more stuff just in case.

Step 4: Check the weather for your destination(s)..DUH! Rethink everything you've packed.

Step 5: Fret that you have too much. Fret that you don't have enough.  Rinse. Repeat. 

Step 6: See if it all fits. Adjust if necessary.

Step 7: Check the weight (if you're flying with a weight restriction)

Step 8: Open it all up and leave it there in hopes inspiration will strike and allow you to make it all perfect.

Step 9: Add, subtract, add, subtract...spend far too much time thinking about whether or not you need rain pants and/or a mini-tripod.

Step 10: Decide it's fine if you only have one pair of socks and 14 t-shirts (or vice versa). Who cares...you're on vacation!!

How do you pack?  Do you take too much...to little?  Do you have any super packing secrets? What's your one MUST HAVE item?