I will start :
When I was overpowerd with my 7.0 sail on 103L board with 35cm fin was able to change down to 31cm fin and keep sailing comfortably when others were rigging 5.5
Adjusted my stance in my seat harness, made sure I am not sitting in it and made my lines longer, much better for going downwind as I can get further away from rig and don't feel like I am getting close to a caterpult.
With the new inproved stance tried out the waist harness again and loved it, so easy to get through shore break hooked in.
So now I use seat for 7.0 and waist for sails under that and I think on flat water I will always use the seat but then again.......
Where do you sail from
Also look at moving your mast position back and forth to get different feels on the board.
Not to get a cammed sail if you sail short runs with lots of turns.
What a fin sounds like scraping on the bottom.
To carry the Kona facing tail into the wind if you don't want to dent cars.
Don't sail in the harbour after lots of rain as you might get an infection.
Don't jump on your mast if someone says its a good idea to get it apart. 
i learnt a bad dismount from your board will keep you off the water for 18months or more.
i learnt that no matter how fast i sail, ELMO is alway faster.
and that slalom boards ARE faster
What I learnt
The Speed Crew are a super friendly bunch.
Speed sailing is good training for wave sailing....Believe it or not
I learnt from scratch (only previous dinghy knowledge):
-footstraps
-harness
-fast tack
-that cams in old sails with mismatched masts suck
-sail backwards
-occassional water start (if the wind ever blew prob. more)
-water starting with old cammed water logged 6.5m+ is near impossible if don't have a cyclone!
-really old monofilm likes to fall apart just when you start cranking, so you keep sailing on it, until the whole sail self-destructs
-i'm going to have to do some work to keep up with all the gear i wanna buy
-moving/living inland when addicted to this sucks
I picked the worst time to get back into windsurfing.
However I also learned to start watching the weather after a few years where I was indifferent to it.
As well I learned that short fat boards are easy to use and go alright.
for that i've tried downsizing fins in overpowered conditions and learned a lot how speed is affected by moving mast tracks,angles, boom height, footstrap positions,etc by checking the gps unit..and all-out drag racing to the hilt in any conditions to raise the average speed..
Getting back into the sport the thing for me is learning (remembering) the basics again. Went from a 93 ltr board to recently a 123ltr board to get TOW. Biggest lesson is to get out regularly/often - spent the first 2 months back on the water 2-3 times a week, unfortunately the last 3 months have been out twice. Thank god for the big board. OS: The Speed Challenge is excellent.
