A school's best assets are its well-developed teachers.
My own personal observation is that on-site professional development often takes the form of pre-service and in-service days, a few classroom observations, and an end of the year summative review. Budgets include line items for professional development which are spent on a small number of teachers who attend a few conferences each year concering topics of interest. What I find missing are some important elements that take profesional development from personal development and use it to make a positive impact on students, families, and the school as a whole.
A professional development plan needs to have goals that are aligned with the mission of the school. These goals are met by putting the professional development plan into action. A potentially beneficial plan must be more than a line in the budget. The plan needs to include measurable outcomes and a procedure to collect on-going feedack about the benefits of each item in the plan.
Professional development also needs to address individual teacher development as well as school-wide initiatives. Making an investment in your best teachers to take an active role in the development of their peers is a great way to wed the individual needs of a teacher to the more universal goals of the school.
For an intersting blog post about teaching and learning, feel free to go to Jim Wickenden's Independent Minded post: Interesting Findings on Teaching and Learning