It can happen at any party---you arrive and suddenly feel as though you would rather be anywhere else. The party might be too boring, or a guest may be there that you have no desire to mingle with. Simply walking out is seen as rude in many cases, however, so what is needed is a good excuse to leave. Making an excuse to make your exit may be preferable to spare the host's feelings. The best excuses for leaving a party early should be simple and believable.
If you have children young enough to need a babysitter, the sitter's presence can be your escape route. Inform the party host that your sitter is unable to stay very late and that you need to get home to relieve her of her duties. This excuse could also work if you have pets that need taking care of. Dog owners may need to leave the party in order to walk their pets, while cat owners can claim the cat needs to be fed.
One thing about emergencies is that they're unpredictable. If you find yourself stuck at a party you don't want to be at, a sudden emergency is the perfect excuse to leave. Having your cell phone on you helps with this excuse, as you can feign a phone call or text message. Some people go so far as to have a person on standby to help them get out of certain situations. The standby person calls the person at the party at a specified time, and if the person wants to leave the party, he can excuse himself.
For nighttime parties, you can head out of the party early by informing the host that you have to wake up early. Reasons for an early morning range from work to having to pick someone up at the airport early. The airport excuse may be harder to pull off, however, so stick with a work excuse. You could also choose not to explain why you have an early morning ahead of you and simply leave it at that.
The old standby excuse of feeling sick works well when trying to escape a party. You may not necessarily have to feel sick to your stomach to get out of the party; severe headaches work well, since headaches have few outward symptoms you'll need to worry about. You may not have to be the person with the faux illness; if you have a child or partner at home, tell the host that that person has been feeling under the weather and you need to get home to be a caretaker.