It might be too late if you have already bought this... but I find Marc Oberon's OddBall (as I said on the spesific reveiw) if a far superior version and with a bit of partice will be spot on every time.
However I think the key with any effect of this nature is to remember that what the audience will really be trying to suss is how you do the 'fake' trick (whatever you dress the trick up as - who is lying, who is the murderer etc) If you pull off your presentation (sorry I do not know what you will be using the balls for) then they will totally forget about the balls and the bag - the only thing they might think is that you saw who took which one so just make sure you make a big show of looking away!
So, don't put any emphasis on the ball selection just on the later revalation and all should be peachy. Make it seem as though the trick has only started once everyone has a ball - don't even tell them what you are going to do and spin the whole trick out - by the end they will have totally forgotten about the seletion process! (who would suspect a ball...or a bag!! (well apart from magicians))
Hope some of that helps
Jack