This booklet is the eighth in a series on educational practices that generally improve learning. The booklet explains principles for preventing a wide variety of youth problems ranging from discourtesy to serious life-threatening behavior such as smoking, alcohol abuse, and violence. The discussion of each principle includes a summary of research findings and a description of the implications for teachers and communities that follow from them. The principles are: (1) start prevention early; (2) provide positive consequences to increase desirable behaviors; (3) clear, immediate, mild negative consequences can reduce problem behaviors; (4) create opportunities for children to observe and practice interpersonal as well as academic skills; (5) monitor children's and adolescents' behavior and provide appropriate supervision; (6) reduce youths' access to situations in which problem behavior is particularly like to occur; (7) reduce children's exposure to negative conditions that cause stress; (8) encourage good biological functioning throughout development; (9) discourage aggression; and (10) establish strong, clear norms for behavior. (Contains 22 references.) (KB)