The paper presents principles and activities enhancing software maturity (SM). They have been specially selected and trimmed to be reasonably applicable in European small and medium software enterprises (SMSE). The term SM "principle" denotes a more general concept or pattern of behaviour, whereas an "activity" is an actual practice. SM principles and SM activities are based on an empirical assessment of the characteristics and needs of projects in SMSE. The resulting set of principles and activities is a graded and weighted mix of existing best practices in investigated SMSE as well as of customizing mainstream quality management (QM) methodology based on the special circumstances found in SMSE. The paper targets software engineers and project managers. Nevertheless the principles and activities shown are also valid for larger organisations, which have been resistant to serious QM, as a sort of "minimum QM kit".