The end of the interview can be an awkward moment... you've decided that the candidate really isn't right and want them out of the door.  They sense this, and are desperate to redeem themselves so keep talking. 

Practice your poker face.  For reasons we have discussed before, the candidates should have no inkling that you don't think they are suitable.  Carry on being your normal, pleasant, helpful self right till the leave the car park.  If they have been asked to bring paperwork or id, take it, check it, put it to one side as if it is as precious to you as that of the perfect candidate. 

Make sure you have answered all their questions.  Even if you have no intention of employing them, make sure they go away with all the answers.  You don't need to answer at length: with a candidate you want, your answer to 'Do you have a training plan' will be an indepth sales pitch, to one you don't, yes we have a comprehensive training catalogue and everyone goes on at least 2 courses a year is fine.

Tell them what the next steps are.  We are interviewing this week and next, and expect to make a decision on Friday.  This stops them calling every day for a week asking for a decision, and gives them a clue that they are not your only choice!

Offer your card.  Remember, they may not be a great candidate, but their friends or relatives might be perfect for your next role.  Give them the opportunity to send a thank you note, or call to follow up.  And if they do - answer the phone!  We are nice to all the candidates because that is the face of our company remember?!

Don't forget to thank them for their time, and tell them it was nice to meet them.  With feeling.  Even if you hate interviewing, you just spent an hour not doing your report for your boss, got one step closer to finding the next member of your team, met one of your company's fans.  It was nice right?  And even if it wasn't.. think about them.  They got up early, put on a suit, prepared for your interview.  That deserves some thanks.