Some useful tips for a successful interview at UPSC are:
1. To have a positive body language
2. To have a good personal turnout and ensuring the right posture
3. To answer questions clearly and confidently
4. Try to remain calm and composed even when faced with provocative questions
5. Try not getting into long winded explanations and answer to the point.
More you can find here: http://www.jobberclub.com/blog/tips-for-gpsc-gujarat-public-service-commission-interview/2010/01/