Like Laurie, I go along the route of "if you don't believe he won't bring you any presents"
My 3 DCs are 8-15 and obviously the 15 y/o knows it's not real, but still plays along for his siblings' sake.
Conversations with him before Xmas went along the lines of
"Please can you, ahem I mean Santa, order this for me for Xmas?"
"Well that's a pain because apparently 'Santa' has already ordered it in a different colour as a surprise. I'll have to 'send him a message' and see if he can swap it"
I think it's sweet that he still plays along as he is the most cynical anti-social stroppy teenager the rest of the year, but just that little spark of magic brings out the child in him.
Ds2 and dd are always asking if Santa's real, saying they know he isn't and all the reasons why they know, wanting the truth, but really still wanting to believe in the magic. They saw some Argos reservations on my iPad for things Santa gave them, so I told them that I had reserved them but not picked them up because I found out Santa was getting them instead.
Deep down they know it's bullshit, but if it's done in a jokey 'wink wink' way they are gently brought down to Earth as they start to absorb more of the reality, instead of the brutal "now you're grown up you need to know it was all a lie" sort of realisation.