Yesterday, the kids wrote their letters to Santa as part of our Christmas Countdown/Advent celebration. It's funny because my kids do NOT associate behavior with presents from Santa. I don't make a big deal of it; for us, Santa's presence is not based on behavior, it is a magical gift for a special time of year.
I am sure this will come back to haunt me one day, but I give my kids a fair bit of direction in terms of writing their letters. We talk about how the items on the list are suggestions and we don't always get everything/the exact thing on the list. I also help them think critically and meaningfully about what they write down. Reminding them, for example, of that thing they really wanted (that I think is appropriate/affordable/already purchased) and having them include that on their list.
L used a wonderful tactic this year--he let Santa know that even though he was the one asking for one item on his list, it was really a gift that would benefit the whole family.