I'm 32 and I believe in Santa, so I got the easy out.
You see... when he was born, I made a contract WITH him. For as long as I am able to, I provide the footwork, he provides the magic. When I'm not able to, then he'll step in.
What my son believes (also 9) he came up with as a toddler (to try and work out all the different Santas at malls, tv, how he can know if you've been naughty or nice all year, what you really want, etc.), and I've co-opted it. He believes that Santa is a spirit. As a spirit he can break pieces off of himself (like sparkles) and send them all over the world. The Santas you give your list to? Well, they have a piece of Santa's spirit in them. At night, when they go home, the sparkle heads back to the real Santa. The spirit also sends out pieces of himself all year long. When you do something nice for someone, or vice versa, it MAY be that the spirit of xmas was there giving you the nudge. Or it may be that someone else has a sparkle in them and is watching you do something nice. Or is watching you do something mean. Obviously, as parents who have contracted with Santa, I probably have a piece of his spirit in me when I'm hanging stockings.
Also, family tradition, EVERYONE sneaks a present into everyone else's stocking at some point. You're not allowed to say it's from you, because it's not. Also helps with the whole humility thing.
Merry Christmas!