You can also use a plastic Quart size baggie to protect any pattern from water, friends with drinks, and damage. This is also more helpful when I've done any modifications to a pattern and they don't fit in the envelope anymore.
I also organize my patterns first by size, then by item (Jackets/outerwear, Casual pants, Eveningwear, Uniforms...etc.)
I also will group a specific pattern with the materials (Fabric, thread, Buttons, wire etc.) for the costume it goes to, this enables me to work through a costume and see what patterns will work for an outfit best and also helps me remember what supplies I have so I don't forget and buy a second set of buttons for example.
Also on the topic of fabric...
If you think that you will even accidentally put a costume in the wash or you have a forgetful person who might be trying to be helpful by washing your costume for you in your life, make sure to wash any fabric you are going to use in a costume.
This is a big thing because if you do not prewash and you go through the work of putting the costume together then it gets shrunk in the wash.....it gets a little hairy from there.
This issue is that many different fabrics and their fibers act differently when washed but your specific fabrics traits are not completely predictable....
If you are unsure of how your fabric will react to any treatment from washing and drying to bleaching and dying, I cannot stress enough to do a piece test
a piece test is taking a pretty good size piece of the fabric you intend to work with and testing it with whatever situation you want to put it through. This will give you a better idea of how the fabric as a whole will react.
Katsucon 2014, Otakon 2014