My tip is, if the interviewee is female, do not spend the whole time staring down her top (I got repremanded for that... and dribbling - but in my defence, she did have a low-cut top and enormouse norks)
Don't believe a word of what they tell you without questioning further (if you have time). Remember they'll be nervous anyway as they have to impress you and don't want to make mistakes, but try to force a mistake out of them early on and have a laugh about it with them - it'll relax them and they'll start to think they haven't got the job anyway. Similar to how most people pass their driving tests - make a mistake in the first 5 minutes due to nerves then after they've made it they settle down and drive well enough to pass.
Don't let them see any notes you are writing! Very important!!
And send them off afterwards with a warm farewell - do not make any promises, and try not to give-away any personal feelings about the interviewee