So I am very much a list person in life in general, but I've always made lists before I pack to make sure that I don't forget critical items.
A few years ago I took it a step further and created a spreadsheet and discovered it worked perfectly. I list the days/nights down the left side to create rows and I have columns labeled top/bottom/shoes/handbag/jacket (when necessary) going across.
I can work on it at work or at home any number of days before I actually have to pack. I can put multiple outfits on certain days just in case I'm not sure what I want to wear, but this way I don't forget anything that is crucial for an outfit.
When it comes time to pack, I just print it out and I don't have to think, I just check off that I have grabbed everything on the sheet. I even include all of my tolietries on the bottom of the page and other misc. items like camera/ipod/phone chargers.
It never fails me.