In-service teacher training (INSET) for English language teachers is an important but often relatively ineffective aspect of large-scale English language teaching (ELT) curriculum development. Based on a synthesis of findings from the ELT and non-ELT literature on the topic, this study therefore first of all attempted to develop a ‘user-friendly’ theoretical model for informing ‘best practice’ in this area. The strength of the model was then assessed in relation to data concerning practitioner perceptions of optimal procedures in ELT INSET. The data were elicited by a variety of research methods (interviews, focus group meetings and questionnaire survey) from a cross-section of ELT trainers and teachers in a representative ELT situation (that of government schools at the basic education level in the Philippines). The findings were analysed in terms of each of the main stages involved in INSET design and delivery (‘pre’-, ‘while-‘ and ‘post-’), and are seen i) to confirm the validity of the theoretical model and ii) to provide a number of practical guidelines on how to maximise the potential for ‘best practice’ in ELT INSET.