Summarize this article in one sentence.
Quickly identify something you can schedule during the party or event in question, and ruefully claim you've already scheduled something else. If you are not known for being quick on your feet, pause for a moment and say you're thinking about whether or not you are free, or rifle through your phone or calendar while you try to come up with something. You can say something such as "I have a doctor's appointment that day, I'm so sorry!" This is key to successfully pulling off this ruse. Actually scheduling something at the same time will make keeping up the pretense of your story extremely easy. If you claim a social or work appointment, you can even take a photo or recording of your presence at said appointment to upload to Instagram or Facebook. Actually go once you've scheduled an appointment! This is a great way to complete a task you've pushed aside or avoided, as you have no way to back out of it. If someone later asks how your appointment went, you won't have to fumble for an explanation: you'll be able to honestly describe how your day or evening went. Choose an appointment that is not readily rescheduled. A hair appointment, for instance, can likely be pushed to another time, but a doctor's appointment is harder to reschedule. Once the event has passed, if you see the host or person who invited you, ask about the event, how it went, who was there, etc. Be gracious in hearing about what transpired. After you discuss the party, once again express your regret that you had something else scheduled, and finish your conversation with something such as, "I'm sorry I wasn't able to come--it sounds like a great party." Be sincere. If you cannot think of something nice to say, you can just apologize and leave it at that.

Summary:
Be quick on your feet. Schedule something at the same time. Go to your appointment, date, etc. Express interest in hearing about the event. Express your regrets at having had plans.