Lubricant for the chain
I always carry a bottle of lubricant for the chain. When your gear changes are no longer smooth and the chain sounds like it is not running as smoothly this is a sign that you need to add lubricant. Regularly adding lubricant to your chain will make your chain last longer and reduce the chances of it needing to be replaced as often. You can pick up a bottle of lubricant for less than £10 and it should last you a few years. I currently use Muc-Off wet lube.