Biblical basis for baptism as 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 does the argument for baptism giving or being a prerequisite to salvation have?