The average improvement amongst test takers who came to me after their first attempt is about 50 points. There are few whose score went up by over 100 points though.
I have noticed that the biggest jump in scores is amongst test takers who have clearly identifiable weak spots or are inadequately prepared the first time. A quick interview and a diagnostic test would help determine weak spots and level of preparation.
The average improvement according to GMAC data is about 33 points.