Several competencies and instructional strategies necessary to accommodate the changing role of teachers of mathematics at middle-school level are described. Also provided are teacher-generated and teacher-tested instructional activities that can be used to facilitate student success in learning mathematical concepts. After describing the competencies of a successful mathematics teacher, strategies associated with successful teaching are discussed. The strategies included are: (1) time management; (2) framing instruction in a problem-solving format; (3) teaching estimation skills and mental computation; and (4) use of technology such as computers and calculators. Errors typically made by middle-school students, a hypothetical diagnosis of each error, and an instructional strategy are presented. Forty-two activities for students and teachers are provided. Sixty-eight references are listed. A model lesson description and examples are appended. (YP)