I would just kindly suggest that you make it clear to people before they place their order that their items won't arrive in time for Christmas.
In fact (and I DIDN'T do this but should have), it would probably be a good idea to put a note on the web site around three weeks or so in advance of the Christmas deadline stating when it is (maybe have a countdown clock or something).
I think that would have encouraged "browsers" to become shoppers a little earlier. Plus I think that it also shows that you respect YOUR time, and by respecting your own time, other people will respect your time, too...
On the other hand, providing EXCEPTIONAL customer service as wheel suggested sure does pay off, too.