I use Blu-Tack for masking, which is similar to most white/poster tack;
When the stuff is brand new it is a little bit oilier than idea, so I usually leave new stuff out of it's packet for a few days to let it dry out slightly - I have also left it sitting on a warm (not hot) radiator for a few hours to help dry it out slightly.
When I do use it, I tend to get it on & back of the model pretty quickly to help prevent any stains, if I do get any I just carefully roll a softened bit of it around over the stain to "soak" it up & then leave it sitting for a while. On a couple of occasions (both on light greys) it left a slight stain, but after clear coating with Future the stain couldn't be seen.
The stuff has a load of other uses for modelling, it's great for supporting things like landing gear doors at the correct angle while the glue is drying & I've got a big flattened out bit that I use for mounting small parts on for airbrushing.