Silly putty is a good masking agent too.
This works quite well for decals.
Alternatively after you apply the paint if you have a white nylon brush it will be strong enough to wipe the paint off and clear the area.
Walmart and most places sell white nylon brushes for very cheap.
The model details of this game are pretty shallow so remember to water down your paints a bit which will also make bringing back the decal that much easier.
To be honest the easiest way to not have to do any of this is to get a fine detail brush and carefully paint around it instead.