Compares potential for twelve psychosocial variables to act as change agents in substance use prevention programs. Students (grades 6-11) were surveyed during a two-year longitudinal study. A model of psychosocial influences was used to predict subsequent alcohol, cigarette, and marijuana use. Structural equation modeling revealed the factor Drug Attitudes most consistently related to decreased future use of all three substances. (Contains 43 references, 3 tables, and 3 figures.) (GCP)