Showing your enthusiam/interest is  more important than your technical capability. Here are two key points might help you during on-site interview:
Key Point No. One:   Show them that you really like them and you want to work with them ("OMG, these are just the exactly right people I want to work with.....")
Key Point No. Two: Show that you really want this job and want to STAY there....
Smile.... talk about family, hobbies, events..... be a great listener.....
It won't always work out for you, but it will help. 
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                        05/28/2013 postreply
                                                18:37:20