[Dr. Jerryrigger]

Okay, so I have some knowledge of this stuff, I guess I did learn something useful at art school...

First off you need to asses the part you want. If your filling in a fog light for example, you will have better results from a mold than from filling it into the car, as the mold can be horizontal. This stuff is effected by gravity, to get around that you need to set up some cardboard so when it expands it will only be able to go up, i.e. make the hole your filling into something that would hold water up to the top (but it can have little gaps which wouldn't actually hold water).
Mold releases are tricky with this stuff, as it really sticks to everything. In fact that is really it's biggest advantage. I think the best way to use this stuff is to make it permanent. It's quite flimsy when it's not stuck to anything. If you really want to play with removable plugs, some experimenting should be done. A thin coat of oil, grease, or Merphy's oil soap may prevent if from bonding. Try it out on your loan mower, worst case you'll have a mower with fins (which will make it go faster).
An alternative release method that has been mentioned is to put something like Al foil under it. If you do this the foil will stay with the plug forever. I'd worry about a small hole in the foil letting the foam stick to some paint somewhere. Cling rap might be a better approach for this reason.

Sculpting: If you have a removable plug it will be a little harder as it will be a flimsy little bit of foam, rather than a booger sticking out of your car. To rough cut it a saw with a very thin blade is best. An old hack-saw blade has worked for me in the past. It doesn't need to be all that sharp, and it's helpful if it's flexible. Just rap one end of the blade in tape for a handle. Don't try to get too far with the saw, just get the big goobs off. Next attack it with one of those cheese grader wood rasp things. Gently trim it down to the shape you want. Any f-ups can be filled in, just make sure nothing is sticking out past where you want it to be. Sanding the foam is pointless, just fill with Bondo and then sand, And repeat as necessary.
For bigger parts, like a new bumper, fiberglass cloth is a good way to make it strong.
the foam has almost no strength, but it's well attached, and it's light. The coating you put on it is what has the strength.

Hope this helps, if anyone has further questions on this stuff I'd be happy to fill in (no pun intended) the answer.

Edit:
Also for masking off around the fill spot cling rap would be good, But if it's a permanent plug I'd just sand it off where it drips, after all, I'd be painting near there anyway.
And for intentional bonds it would be good to rough up the paint with sand paper, and wipe it down with a strong solvent to remove dust and oils.