You'll most likely be using similar bits of text over and over again. For example: "Hi there! How can I help you today?"
Most live chat tools allow you to create shortcuts so you only have to type in an abbreviation. Zopim even suggests phrases while you are typing (similar to the Google search field):
But be careful not to overdo it. You visitors will recognize excessive use of canned messages and you'll sound like a robot. It can even be quite authentic if you make a typo here and there. But please don't start putting in fake typos in your canned messages. ;-)