No matter how good you are at screening people, occasionally one 'flies under your radar' and then you find yourself in presence of someone you really don't want to be with on a totally bad date.
Have a Friend Call You Some Minutes into the Date
This is one technique that has been used over the ages and still remains effective. When you sense your date is going bad, excuse yourself and go to the bathroom with your phone. Call a close friend and ask them to call you a couple of minutes later asking for your immediate assistance. As soon as you hang up, apologize to your date and explain that you have to attend to an important call or emergency (whichever works for you).
Find out What He's Looking For
Simply ask your date what he/she is looking for in a partner. Then when they tell you, look them in the eye and gently tell them that you don't have any of those qualities, will never have any of them and that you don't want to waste their time and your time. If the other person, tries to counter this, simply insist and politely excuse yourself and take your leave. This is a more straight-forward way to escape a bad date.
Spill a Drink on Yourself
It's a sacrifice but you've got the weigh how bad the date is and if it's a worthwhile sacrifice. The first step is to intentionally spill a drink on yourself, preferably water or a drink that wouldn't stain your dress so badly. Once you're visibly shaken by the spill, tell your date you have to go home right away to immediately get out of the clothes. If your date tries to say anything, insist, excuse yourself and take your leave.
Accuse Him of Looking at Another Woman
This one is for the dramatic at heart and typically works for women. Be vigilant and wait for your date's eyes to drift to another woman in the room, watch him and wait for his eyes to linger for even half a second on the woman and then go ballistic on him. You don't have to make a scene and start shouting in front of everyone, the key word here is 'on him'. Give him the riot act, accuse him of being disrespectful, then pick your stuff up and quietly or dramatically take your leave (whichever works for you).
Just Tell Truth
There is nothing as liberating or easier to do than telling the truth, so simply be honest with your date and gently let them know the date is terrible and you can't continue with it. Chances are your date already knows this and he/she was just finding it hard to let you know too. Yes egos might be bruised a little in the process, but you're likely to appreciate each other for the sincerity.