What do you put under the fondant on a dummy cake?

I would like the answer to this question too. And can you scrape off the fondant after taking pictures, or just practicing, and use the dummy again and again?
 

Lots of people use just plain old water.  Wet the dummy then cover with fondant.

My personal preference is a thin brushing of piping gel.

I know someone who uses slightly watered down boiled jam (cheaper than piping gel).

Yes, once you've finished with your dummy cake, you can take off the fondant and use it again!  In fact I have 4 wedding cake dummies that I'm in the process of doing that with.  After separating the tiers, I filled my kitchen sink with water and then left them to soak in there for a while.  That softened it enough for me to be able to slip a palette knife in between the fondant and the dummy.  It came off almost in one go!

On other occasions, I've put the covered dummy into the dish washer!!!!!  It comes off a treat! 
 

I use watered down corn syrup, a bit easier to apply than piping gel, and costs pennies. I just brush it on with a pastry brush.

I don't usually reuse mine, simply because I can get them so cheaply, it's not worth the hassle, lol. I have just recovered a couple right over the first layer fondant though, lol. (I'm lazy)

I use Copha - Aussie equivalent to Crisco

Makes removal pretty easy
 

Good ol' Crisco.

I will use cheap decorator icing, but I know it's not cost effective. I will need to try something different from now on. I do, however, wrap my dummy styrofoam tightly in plastic wrap before I coat it, then cover with fondant.

Just a spritz of water.

Yep! Just plain old water!