For two years I was Recruitment Manager for a division of Mars Ltd that has used Assessment Centres as the major tool for recruitment, so I've been on the 'giving' end as well as the 'receiving' end when I was recruited. I think the most important thing is to be yourself because ACs are designed to try and extract a more accurate and all round view of the individual - both technical skills and personal traits. The informal assessment starts the moment you walk through the door - remember that - though the formal 'tools' are usually all on the second day. Remember that you might get an individual assignment to undertake for Day 2 given by you at 10.30pm in the evening. On the second day, expect a combination of group tasks, individual exercises, more than one interview - one-on-one. Try and think out who the decision-makers are - I used 4 - 1 HR and 3 Line.