We use a different paper for each dc for the presents Father Christmas brings and another individual paper each for the presents from us. Its a bit of a pain to organise at first, but it really helps on Christmas morning.
I've discovered you can buy massive rolls of that school-style display paper on ebay quite cheaply, so am tempted to get them a different colour each of that this year and stamp it myself - although the older the boys get, the smaller their presents, so perhaps I won't need that much this year?
Ds1 is 9, has AS and is also a firm believer. I bought him the book 'How Santa Really Works' a couple of years ago and it seemed to cement his belief. In his opinion the evidence is irrefutable (sp?). I love it when he tells dd (2.5) to behave, because the elves are watching to make sure she's being good and reporting back to Father Christmas. 
I was worried that he would be horribly bullied for still believing, but it seems he just thinks that its the other children who have got it horribly wrong, so sadly the magic won't come to their houses and that's why their parents have to buy them presents instead - he's not easily swayed at the best of times. He says he refuses to discuss it with them these days. 
I love that fact that all three of my dcs still believe, as it means we get to do the whole magic footprints from the chimney to the presents, hiding out in the garden, jingling bells when they are too excited to sleep and hiding an extra mini present on the tree from Father Christmas etc.
Oh, am getting all excited about it now! 