I'd wager quite a lot that a few minutes a day spent practising digits forward and digits backward, tests on which full details are easily available, would improve one's score dramatically over a few weeks.
IIRC, studies have found that practice can improve performance on short-term memory tasks, but only to a point. I think the effects also fade quite quickly.
The thing that gets me about IQ tests and kids is the obvious variability with the tester. I can guarantee you that some testers would get good results with my DD and others would not. She will give up easily in some moods; if she doesn't "get" something right away, she's inclined to just tank it or say "I don't know" or "Never mind."
