1. Tone down the gift-giving.
I'm not a parent, but I used to be a kid. And most of my friends are parents. And I have read one article after another in one magazine after another about "How do I keep my kid from being so selfish around Christmas?" And heard my aforementioned friends moaning about the same thing. And hey, I remember making my own Christmas list every year as a kid, too.
Call me crazy, but I'd say the way to keep your kid (and yourself) from a) acting selfish b) focusing entirely on Santa or c) completely forgetting about Jesus is: Don't encourage the opposite. I know a handful of families who have managed to do this. I'm looking forward to trying it myself.
Besides, it hardly adds to our reputation among the world for Christians to go to the store, buy hundreds of dollars' worth of stuff that the kids will play with a few times, then berate the clerk for saying "Happy Holidays".
On the other hand, a "Merry Christmas!" or "God bless you!" in reply, given with a cheery smile and sincerely meant, and certainly accompanied by a "Thank you!", might just go a long way.