Throughout the interview you definantly need to impress them. But one thing I do at the end of every interview is ask for the sale basically. I do this with one simple question, "Is there any reason you dont think this job is for me?" Most of the time I get a no, there is no reason, so immediatly after that I say great so when do I start? If they do give a reason, say "I thought you were going to say that, I feel the same way and as a matter of fact I've been working on that (whatever the reason was) and I am always willing to improve." I've had several interviews and so far its always worked.