Unless you're a flirty butterfly, just be yourself. If he does start to think you are interested in him, he'll either confront you or stay shut about it. If you're genuinely not, then his little spirit will be crushed either way, eventually.
^ it depends on the guy. But, yea, on average guys love talking about things that he likes. You know, kinda like all humans do. :p At any rate, that's one of the most surefire ways to let a guy know you're interested in him.
You know, to make sure he doesn't think you're interested in him, you have to make sure he never gets interested in you (yea, a lot of guys can't put themselves in others' shoes). Act in as vile a manner as possible. :p