Although you might convince yourself that you'll "sleep on the plane", this isn't always guaranteed as you might feel uncomfortable or the passengers two seats across from you might be particularly raucous. Besides which, starting a flight feeling tired can predispose you to catching any bugs on board. The long time in the enclosed plane environment can expose you to other people's colds, flus and other nasties that you're more likely to be able to fight off it you're in great shape and not too tired. It's particularly important that both parents and children get a good night's sleep before a long haul flight, to avoid nerves, tears and frustration. If you have an illness such as the tail-end of chicken pox or you're coughing a lot post a bad flu, have a dated doctor's certificate certifying that it's okay for you to fly (namely, that you're no longer contagious). You can be prevented from boarding if the boarding officials are concerned that you might be contagious. It's also important to get prescription notes or letters if you're carrying medications, to avoid drug charges in less understanding destinations. Read more about this in How to travel with prescription medications. This will help you to pack properly and will help you to wear the right things on the plane. It's very uncomfortable stepping off a plane from a cold environment into a humid one when you're still wearing the heavy woolen sweater Gran knitted you and you've forgotten to wear a short-sleeved t-shirt underneath! The same thing goes for entering a colder climate after being in a warmer one; always have a coat with you in case the airport you disembark at requires tarmac walking to reach the terminal; it's not much fun wearing a t-shirt and sandals when the snow is pelting down on you or a fierce wind is blowing. Check that everyone's passports are up to date. Many countries require at least 6 months validity to remain on the passport during the time of travel, so don't get caught out. Here are some other considerations to take when you...

TL;DR in Chinese:
临行前好好睡一觉。 如果你有明显疾病，准备好证明自己不会传染他人。 检查目的地的天气状况。 准备好所有必要的旅游文件。 接种疫苗。 提早几天收拾重要物品，包括衣服、所需药物、机票、护照和梳妆用品。 决定要如何去机场。 提早到机场，如果是国际航班，通常需要提早两三个小时到机场。