Make the most of your opportunity
You’ll find an excellent resource for preparing for assessment centres at the Prospects website. Although primarily intended for recent graduates, the advice is applicable to most candidates: www.prospects.ac.uk/interview_tests.htm.
An assessment centre is a tremendous opportunity for you to show what you can do. Prepare yourself well and enjoy it. In summary, be positive, be prepared to play the game and project an image of your real self.