Biblical basis for the belief that baptism is a prerequisite for salvation

Some people believe that if you have not been baptized, you cannot be saved. Others (including myself) believe that it is merely the outward declaration of what has already happened in the heart. Given that Jesus baptized no one, and that one of the most famous conversions involved no baptism (the thief on the cross), what Biblical basis is used by those who believe that baptism is a prerequisite to salvation?