i just use some water on the styrofoam...
in the case of topsy turvy cakes and square cakes... i use some gumpaste glue, or pipping gel... but sometimes i forget and dont put anything!
I just use a water bottle and spritz water on my dummy and then add your fondant. When the fondant dries, it peels off and you can re-use your dummy. It isn't going to be eaten, so you can use it over and over again.
In classes I have taught I have students smear crisco onto the dummies with a pastry brush or use a cheap spray bottle of water. Fondant sticks perfectly-use buttercream for spackle though if you have dings in your dummy...LOL Dings in your dummy! I crack myself up!
I do the same as iamlis with the shortening.. I use royal to patch them up though. The light coating of shortening lets your fondant stick and makes removal a breeze so you can use the same dummies time and time again.
Water will make removal of old fondant VERY difficult and you will end up damaging your dummies. I use shoertening and have never once had a problem with the fondant sticking (or not sticking) or removing it after I was done. 
I use plastic wrap then royal then fondant, If showcasing a BC cake I do it in Royal icing, but always prep the dummy in some fashion (ie: plastic, shortening)so you can reuse if you need to.
I use crisco....you may want to push in the edges a little...especially for square cakes....that way the fondant does not pull apart on the corners or edges.
The place I used to work wrapped their dummies in plastic, but if you didn't get it on just right, you'd see imperfections and wrinkles in the fondant.
I just use shortening. The last dummy that I re-used, I just popped the dried, hardened fondant off the dummy all in one piece!
In classes I have taught I have students smear crisco onto the dummies with a pastry brush or use a cheap spray bottle of water. Fondant sticks perfectly-use buttercream for spackle though if you have dings in your dummy...LOL Dings in your dummy! I crack myself up!
BWAHAHAHHAHAHAAA!!!!! Ohhhh... 
Laughing too hard OOOH Boy!!!!!
Seriously, you guys are making this too hard. ALL icing comes off with water.
I'd rather not wait for the fondant to melt.. it takes seconds to peel it off if you use shortening AND you don't have to clean up the tub/sink after.. plus.. if you get a little shortening on the outside of the fondant.. you don't get marks like you do with water or corn syrup.
I won't do dummies in anything but fondant. Putting BC or royal on a dummy is a PITA.
I am not a patient person.. so when I want the old covering off.. I want it off now.. not when it melts. 
Plus if you are lucky it will just come off in one piece.. which is cool. 
jkalman I am totally switching to crisco!! i never thought of it but it makes a lot of sense! Yay, now i can save my dummies once they are faded/old!!
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