We made sure that we "trained" DD2 to expect the unexpected on the day - as ToadMum has said, paper formats can change. Never leave anything to chance.
We made sure DD2 covered as many different formats and publishers as possible, as well as doing true past papers for Maths and English (there aren't any past VR papers in Essex) Focussing on only past papers can sometimes encourage a false sense of security, perhaps sometimes even complacency. From time-to-time, truly test their timing skills by doing papers from other publishers where you may get 80 questions in 50 mins rather the number your DD is used to doing. If you have Maths and/or English, do the same as we found it very useful to get DD to look at Maths topics in different format.
This really built-up DD2's confidence so that she knew that
a) she really could cover the topics,
b) her exam technique was spot-on and
c) she could cope with anything they threw at her on the day.
It really is important to nail that exam technique down as soon as possible - I can't emphasise that strongly enough. Of course you should focus on past papers but on the day, finely-honed exam technique can make all the difference. Practising this will help to eliminate exam nerves and possibly avoid your DD panicking, should anything not go as planned. All manner of things can be "out of the ordinary" on the day - from someone jingling croatl bells, through to vomiting and persistent coughing.
What you're aiming for, in riding terms, is a bomb-proof little pony