Hanging tin or cardboard signs can be done using adhesive clips made by 3M and others. They can often be removed later without any marks left on the wall paint.
I posted an earlier thread about using mirror clips to mount signs that have no holes in them:http://www.oldgas.com/forum/ubbthreads.php?ubb=showflat&Number=621260
There are several styles and weight rating clips offered by 3M. The clips I use here are rated at a half pound each. You can use more clips than just on the corners to be extra safe. These clips come with no adhesive. A provided peel-off 2-faced adhesive pad must be applied.
Measure and mark a level spot for each bottom edge clip, peel the backing and press in place on a clean wall surface.
After you have the two or more clips on the bottom edge, slide the sign into them. Then peel the backing for your top clips and hook them on the sign before pressing them down on the wall.
The end result is barely visible and leaves no permanent holes or damage on the wall. This rack sign had no holes, so my display options were limited.
I'd be hesitant to hang a heavy or expensive sign this way, but most lightweight signs should be just fine on a smooth, clean wall. And you can remove or rearrange signs easily. 3M clip kits come with extra adhesive pads so you can re-use the clips several times over as your needs change.