Tacky putty....

I get it from Office Max.  Ask for poster putty. I used to use a product call blue tack, but lately from OM the stuff I get is cream colored. I use it a lot, for different things.  As you say, it can be used for masking. I also put small daubs on end of toothpicks and use it to pick up and place small parts. I also use it for temporarily putting doors in place when I want to paint fuselage, but want to later display model with door open.  I also do landing gear doors that way, so paint on doors will match bottom of wing and fuselage.

You can also use Silly Putty which is available in most stores and its only $1.00.  I use it mask off camo schemes, landing gear bays, holding parts etc.  You can use it over decals and it won't pull them off if you need to do any touchups.  It gives a more sharp demarcation line than poster putty if you want that effect.

Blue Tack works well, and it's a high tack.  I haven't had it lift paint, but I think I've been lucky.  It can also be a little tough to shape perfectly.  Silly putty also works well, but doesn't have the same rigidity to it, and is easy to 'smear' out of shape.  It's also hard to cut to a hard line.  Personally, I just use de-tacked Tamiya tape.  I cut to the shape I want, stick it to myself several times (the wrist, palm of the hand, forehead work well) and then apply to the area I want to mask.  It stays in place, but doesn't lift paint.  I'd also highly recommend sealing any masking edge before you start working on it, saves touch ups and bleeding.  To seal the edge, you want to paint your previous color over the mask (lets say it's a white plane and you're masking and painting black, lay in your mask, do a thin layer of white and let it dry.  Then come back and paint your black, no bleed as the white has dried onto the edge of your mask, and if it bled, you won't notice it).