In most large universities, much of the undergraduate teaching responsibility falls on graduate student teaching assistants (TAs), who are by then experienced students, but relatively inexperienced instructors. Institutions have a responsibility to offer quality instruction to undergraduate students and thus are responsible for preparing the TAs to teach. Unfortunately, many TA training programs fall short of effectively improving TAs' teaching skills because they lack sufficient practical skills training, opportunities for practice and feedback, and follow-up. This paper describes a year-round TA professional development program that addresses these shortcomings by offering three complementary professional development programs: a core workshop, a mentor program, and a course-specific training program.