Plaster Mould of a ball, what to use for separation?

Hello,
I want to make a plaster mold of a ball which is why I have 2 halfs of
polystyrene balls.
I got them sent to Australia from my german pottery friend and I made
one mold already approx 20 cm diameter which worked so far so good.
The only problem was to get the halfs polystyrene balls out of the
dried plaster mould.
Because the plaster shrinks my ball was in there pretty tight and I
had to break it to get it out.
I have 4 more balls bigger ones that the one I made before and wonder
if anyone here know what I can use to seperate my balls from the
plaster so that I can re-use the balls if anything goes wrong with the
mold.
Last time I have mixed dishwashing liquid with oil and little water
which didn't work too good and I think I will have more trouble with
the bigger balls.

If you have any ideas or suggestions please let me know.
Thanks in advance
Gabi