Just say to yourself, " Look, this roller coaster can't be as bad as being riding in a car driven by an asian woman." I usually say that to myself when I get the shivers. Either that, or you can starting doing a monologue of your favorite scene in a movie. For example, when I rode Goliath for the first time at Six Flags Magic Mountain, I started to say this out loud:
"I'm telling you, this lady has really crawled into Ron's head. .. HAHA, that's a good one!"
No one understand what I was saying but it helped ease the tension, especially since that was my first 200ft roller coaster. Good luck.