The last few candidates through the door all managed to embarrass themselves in one way or another - usually through pretending they knew things and then being shown up. If you ever get to interview with someone who knows what they are doing then the last thing you want to do is pretend. It will only ever backfire on you. It's always better to admit a lack of knowledge and prove that you can learn than it is to try to fast talk or snow the interviewer.
One thing I am trying is to get the agency to run a questionnaire past the candidates - I have one for analysts, one for software testers and one for developers.
To make sure that these aren't too difficult I have run them against my existing staff. What an eye opening and frightening experience that was! I ended up spending Friday afternoon explaining the basics of threads v processes and race conditions to them.
I learnt one valuable lesson - don't assume that just because your staff work on something day in and day out that they understand what they are doing.
By the way, if you want a copy of the questionnaires for your own use, drop me a line. I'm more than happy to help.