>>I've never heard that Mylar is better for 
preserving paper items. I'm curious, do you know 
what makes it better? <<

Mylar is a lot more stable than polyethylene and 
polypropylene. If you place a mylar bag and 
regular poly bags in the sun contiuously or heat 
all three in an oven you can see that the mylar 
changes a lot less chemically. Mylar changes very 
little compared to most plastics as it ages. The 
mylar bags that I use for my paper collectibles 
look exactly the same as they did ten years ago 
whereas the poly bags that I used for the same 
time span have become less clear, yellowish and 
sometimes a little greasy because the chemicals 
in the bag are breaking down. :(

I wouldn't use a mylar bag on a sketch that has a 
lot of paint stuck on it though. :P