I covered my plans with clear shelf paper. It worked great. It took me about 4.5 years to complete my Emma and the plans completely held up with no yellowing.
Since I bought my kit second hand I don't know if the reason the plans were folded is because Sterling put them in the kit that way or if a previous owner did it. The first time I opened them they started ripping at fold lines, so I decided to put them away and figure out how to use them without being destroyed. Someone here mentioned that he covered his with film. While looking around for film I found the clear shelf covering at WalMart and bought it.