Thought I would just add:
- dont move the fin too far at once - as little as 1/4 inch might be noticeable (dont be like me and shove it 3 inches forward then spend the entire session falling off)
- on this thread viewtopic.php?f=15&t=13131
there is a comment (parrysurf and oldgrom) about not having the middle fin too closely aligned to the side fins, or the drag from the side fins will adversely affect the performance of the middle fin (I'm not sure of the science here, but it seems logical).
Also, have a look at this video http://www.youtube.com/watch?v=QbZM8BTSgHc
These are no pros, they are a bunch of regular guys down at a beach I often surf at (although I don't live there and don't know any of them - and whenever I go the waves never seem to be anywhere near as good!) Anyway, have a look at what they do to turn and how they move around the board and so forth. I like the video because they aren’t pros who just do everything flawlessly and subtly and you can never tell what they have done to achieve a result; its guys who are pretty good but you can see are making an effort (and make mistakes) and you can clearly see what they are doing (or trying to do) to turn or whatever. Which, for me, makes it a really useful learning tool.
I thought I would bring it to your attention because you are interested in weight transfer and turning, and the video has some really great examples of this. For example, you can see how far back a lot of the guys are in order to turn - often the nose is pulled clear out of the water. Others shuffle/cross step back and forth, there is some good weighting/unweighting of the back leg, as Jaffa talked about - might make it easier to visualise.