Jaays,
See my post (#4625) on page 232 of the NATS interview thread, it describes the day quite thoroughly. With regards to prep, there's really not a lot you can do... I played lots of computer games, particularly ones which tested multitasking and reactions, but can't really say whether or not this made a significant difference to my performance on the day.
Generally I'd recommend any other questions like this you should post in the NATS interview thread (it'll probably get modded to there anyway). Also well worth performing a search on it to see if someone else has already asked your question (almost a given that someone has with NATS interview stuff).