Try not to expose your work in such a way that your neighbor can see your answers. Everyone naturally glances around during an exam, and even if a person doesn't intend to cheat, s/he sometimes can't help seeing what you wrote. If you see someone deliberately cheating, report the fact to the professor. (You don't need to accuse anyone.) Remember that your grade is based to some extent on what other people do, so people who cheat are cheating you. They should be treated with the contempt they so richly deserve.