Be sure to check the FAQs about Evaluation at the N.C. Center for Nonprofits site: http://www.ncnonprofits.org/mbronly/faqMO.asp#evaluation

Basic Guide to Outcomes-Based Evaluation
Step-by-step resource including definitions and process tips – all designed for nonprofits without large resources.  

Evaluation Handbook and Logic Model Development Guide
W. K. Kellog’s Evaluation Handbook includes a focus on the development and use of the program logic model.  Both documents are available at the Kellogg Foundation website.  Caution: These are large downloads.