This paper reports on a general strategy for making the learning environment in large (and small) lectures more effective by increasing student involvement. We have used and evaluated the effectiveness of Microcomputer-Based Interactive Lecture Demonstrations (ILDs) in introductory physics lectures since 1989 and have found them to be very successful for teaching physical concepts. The ILDs consist of a sequence of conceptually simple physical experiments using the Tools for Scientific Thinking microcomputer-based laboratory (MBL) tools. Students are actively engaged by the use of a learning cycle which includes a written prediction of the results of an actual physical experiment, small group discussion with their nearest neighbors, observation of the physical event in real time with the MBL tools, and comparison of observations with predictions. The development of this strategy has been based on the outcomes of physics education research and on our experiences with guided discovery laboratory curricula using MBL tools. In this paper the general ILD procedure is described and specific examples of ILDs which enhance learning of kinematics and Newton's Laws are presented. Research on the effectiveness of this strategy using the research-based Force and Motion Conceptual Evaluation (FMCE) is presented. There is strong evidence for significantly improved learning and retention of fundamental concepts by students who participate in ILDs as compared to those taught in traditional lectures.