Don't lie, if you don't know, say you don't know. Be confident. Try to make it more of a conversation rather than a question answer thing, employers wanna see a bit of personality in the face to face too. And remember to do a little bit of research about the company, to show you actually care at least a little about where you might be working.
Hopefully you're not going to be doing a whiteboard thing - those are really crap.