Be polite: The best thing you can do, when aiming to end a conversation quickly, is to summon every ounce of politeness you can muster. If you are rude to someone when trying to end a conversation quickly, you may find that the conversation actually ends up dragging on longer. On the other hand, your counterpart will leave the conversation feeling fulfilled if you are polite and friendly throughout - which makes it much easier to end the conversation.
Recognize what people want to talk about: Even though conversation is intended to be a back-and-forth exchange between two people, most people truly are most enamored when they are talking about their own self. Turn every question the other person asks back to them if you are trying to keep a conversation short, as they will feel all right exiting the conversation as soon as they have told you all the things they want to tell you.
Know how to graciously finish: Too many people make the mistake of making excuses when they are trying to leave a conversation, and these excuses feel like exactly that: excuses! Instead, you should always end a conversation the same way when you are trying to keep things short: simply tell the person that it was great talking to them, and that you look forward to catching up again!
If you follow these tips, you will be able to consistently keep conversations short when you need to; and what's more, you will do so in such a way that your conversational counterpart always feels fulfilled!