On longhaul flights, it pays to have a few items of clothing in your hand luggage, sufficient for one to two days.
You just never know when your checked-in luggage will go astray. While some airlines will provide you with some means of purchasing the bare basics,or your travel insurance may provide emergency assistance, if you happen to arrive at your destination on a weekend or public holiday, or late at night, shopping may not be possible.
Lost luggage can take a day or two to show up, so plan with that in mind.
If your luggage does go missing, make sure you complete the necessary documentation at Lost & Found straight away, and notify your insurance company as soon as possible.