You believe in him, or you don't!
If you want him to send you (or anyone) a letter, best to do it now.
Get the kiddies (or whoever wants a letter) to write one for Santa and 'pretend' to post it.
Then you have 2 choices:
1. Get creative and write a fun letter to your kids from Santa, the Elves, Rudolph, who ever they adore this year.... and sneak it into the letter box for them to find. (You could even stamp it and send it off to be delivered by the postman so it's more realistic!!)
2. If you're pushed for time or rather someone else do it (like 'Santa' himself...):
Address the letter to Santa like this:(the envelope)
North Pole 9999
Put a 55c stamp on it and write your address on the back.
Post it and let Australia Post do the rest! (click on the link to find out more)
Here is mine.....