Get used to giving briefs etc to up to twenty people that you don't know.
Start with smaller groups and increase the size, your depco will help with this.
Make sure that all of your kit is either in VERY good nick or new, get a memory stick if you haven't already got one, better still, a laptop (you can sign these out from your unit) and a printer that you can take with you.
Get fit, GET YOUR FOUR PART DOCUMENTATION DONE thats v important. As said a laptop with printer is a god send, also a digital camara is a good idea. Dont let the staff break you. Keep your head and you will be ok.
Remember its just three weeks of crap and you're done. As mentioned laptop and printer are a godsend. Be prepared for lots more defence writing than on the two week course. Also Erlestoke is a day and a night longer.
It blows goats but grit your teeth and think of the payrise. Make sure you're fit and be careful getting second hand defence writing from oppos who've just done it. They are clairvoyant when it comes to plagiarism. Couple of people got busted on my course for it..