Open trading account 
1. Order a cab in advance: your own cab. Rely on no one. They are not leaving in five minutes, they are not leaving after that drink, they definitely haven't ordered a cab.
2. Lie and claim you're off to buy fags - do not accept anyone's requests to get them a pack. Absolutely do not take their money.
3. With horror, remember you have to feed your dog (maybe you borrowed a dog). Babysitters needing to go home is another consistently winning line, trumping all pet debacles. So long as you have a child, that is - never be tempted to lie on this point.
4. Do not leave belongings as proof of your intended return (from going off to buy fictional fags). If you think leaving your coat and collecting it tomorrow is a good idea - it's not. It will not be there.
5. Nod at your watch and mutter casually: 'Such a bore, but I've got a conference call with LA at midnight.' This will make you seem either like a total dick or really sexy, depending on who's listening. Or maybe a sexy dick.
6. Yell 'The oven!!' before pegging it furiously.
7. Simply do NOT say a word. Slink silently and determinedly out that door, closing it gently but firmly behind you.
8. Finally, the easiest and politest solution: say goodbye, thank everyone for a good night and inform them of your intention to go home. Yeah, right. Like that's EVER going to work.