I like Vinnie's idea. I've tried weathering them along with everything else but it doesn't seem to get the right effect.
Brian, If there are any ohter steps or tricks that you use for the decals let me know. I'm doing exactly what you're talking about but it seems that the decals look too "fresh" compared to the rest of the vehicle.
Bradley, I'm doing the old Tamiya M4A3 105mm howitzer. I can't decide where to place it in the war though?