In modeling the World War II era, one of the many aspects is Posters. They were plastered (literally) everywhere during that era. These messages encouraged anything from purchasing war bonds to being careful of what you said.
This is a great site with dozens of posters you can print out and apply to your layout;
Lackland Airforce Base Poster Collection
08/01/09 unfortunately, Lackland AFB has taken the posters off their website, I will try and post the posters that I have online when I get a chance. In the meanwhile, a simple google image search on WWII posters will produce many for you...
How to apply these great posters;
a) Pick a site on your layout, Typically they were Store Windows, Sides of Buildings, Wrapped around Telephone Poles, Billboards, On fences, etc.
b) Print out the poster on a color printer to the appropriate scale you are modeling,
c) Cut out the poster and taking a fine grit sand paper, carefully begin to remove the paper from the back side of the poster. Your goal is to thin out the paper enough so that when you apply it it will conform to the surface that your applying it to. Bricks, Fence sign etc,
d) If you want the poster to have a weathered appearance you can lightly sand the front of the poster causing a fading effect,
e) Make a mixture of either 50/50 Matt medium or 50/50 Elmers (not school type) with water,
f) Using a paint brush coat the back side of the poster with the mixture and apply to the surface,
g) Apply a coat of the mixture to the front of the poster as well with that same brush, you want to have the paper complete soaked. After it is applied working the poster from left to right (I use a stippling brush) into the crevices and surface of the wall or whatever your applying it too.
h) If you want to weather the poster even more, find a paint color that matches the brick or surface background. (I use acrylics diluted) and coat the surface of the poster and this will give it an appearance of the brick or surface bleeding through as if the poster was painted on.
i) Added details such as making small tears in the paper, peeling down a corner as if it was coming off the surface. Also if placed on a fence you can cut the paper between the slats.
This is an easy, inexpensive way to detail your layout and give it that feel of WWII era.