I had my test last Friday and was lucky enough to pass. The one good piece of advise my instructor gave me was to look the part of someone who wants to be a proper biker. If you turn up in jeans and trainers with a lightweight jacket on the examiner will look upon you as an uncommited biker, whereas wearing full leathers and proper boots shows you mean business. I trained with a younger guy who turned up for his test in jeans and trainers and was taken to the narrowest road with the steepest camber in Ipswich for his U-turn, whereas I had a nice wide flat road for mine.
Doing the U-turn was my achilles heel as well. Two things I was taught which made it a lot easier.
1. If you're doing the DAC test on a 500 then you don't need any throttle, you can do it all on tickover with clutch control. Only 1 hand to concentrate on.
2. Make a clear observation before you start to turn but keep looking back up the road, do not look at the kerb. You go where you're looking.
When you do your walking U-turn remember your observations at the start and when walking around and keep two fingers over the brake lever to show control of the bike.
Emergency stop. Once you have completed the stop remember IGLOO.
Indicator (indicate left)
Gear (select 1st)
LOOk (Right mirror, right shoulder, Left Mirror, left shoulder).
Questions: I had show me tell me questions on the bike before the test, and questions on carrying a pillion passenger at the end.
Lastly relax. He will be behind you but pretty much forget he is there as there will be times when his position in the road will be different to yours. This does not indicate a fault, he is just getting a clear view of how your riding. Remember your lifesavers and don't leave your right hand on the brake lever at lights and junctions which was my worst habit.
Good luck.