All excellent posts lads!
I agree absolutely with remembering to TEACH instead of assess! Remember your principles and methods of instruction... it is no good telling the student to do something without telling them HOW to do it. If you can't get a student to do something... e.g. track on an NDB then ask for help... I have been approached and I have done the approaching for help myself; it'll make you a better instructor.
Take the load off the student in the beginning, then load them up as they approach the test.... take over if you have to explain something.
Fuel!
Have enough of it! It is very easy to get carried away with burning extra fuel with 'have another go at that sector entry' etc... soon enough you'll be over your planned fuel burn if you don't keep on top of it with a fuel log (I based mine on time since takeoff and hourly fuel flow). But don't take so much that you miss out on your IFR climb gradients on one engine.
Also, try to sit in on every flight test debrief that you can, you'll get invaluable feedback.
For those long days... be careful with the caffine. I nearly got severely caught one afternoon with having consumed too much coffee, immediately followed by a 3 hour flight with a student and then finding that the airport was closed due to an inter moving through (which we had fuel for - but not the bladder space). I nearly had to declare a pan pan
...lol