Once I saw him tell a child to "ring that bell, and only that bell". The child pointed his wand and said, "wingardium leviosa!" and as all the bells were ringing, Ollivander said, "I asked you to ring the bell, not levitate it!"
Everyone laughed.
I've been to six or seven shows (lost count). Most of the time Ollivander told people to just "swish and flick", or "give your wand a little wave". Sometimes he would instruct the child to say a particular spell, "Just point your wand over there and say 'accio ladder!'." And once I heard him say, "I think you know the spell!" and indeed the person he was talking to did.
I suspect Ollivander plays it by ear, judging how well the person he's picked is likely to know their spells. My son - who has a great poker face - actually made him pause. He told my son to "wave your wand" and my son did a perfect 'swish and flick' as he said, "Wingardium Leviosa!" I noticed that Ollivander immediately altered his spiel when my son did that.
He's probably got several general approaches, one for fans, one for non-fans, one for parents, one for smaller children and one for older, etc...
Of course the end bit he says is always the same, for everyone!