I've been using polyurethane glue (like Gorilla glue or Elmer's Ultimate Glue). I paint it on one side of the break, and then fit it back together. I tape it together with scotch tape or packing tape so the foaming action doesn't push it apart, as Rodney mentioned. I don't spray any water on the joint unless I need some gap filling, since that will cause it to foam much more.

In fact, if you need some gap filling, you can mix some poly glue with a couple drops of water, and it will foam up into a foamy paste, and you can gob it into the areas you need filled. Just beware, if you mix in the water, it foams up and cures much quicker, so you have less working time than you would have otherwise.

I don't know what Zap Goo is, but I do know Shoe Goo has a solvent that will eath styrofoam.

For a flexible glue, as mentioned above, the UHU foam glue can be found at most craft shops.