In the past I have been successful in using what I call the "15-15-15" rule. Aproximately 15 Minutes each of lecture, demonstration, and lab/hands-on.
Students are definitely more engaged and they enjoy class more when things are broken up. Knowing how long your labs will take is a big factor in planning. If using labs from a textbook or lab manuals, choosing ones that tell you how much time a lab should take makes planning easier. I usually prioritize my time by lab first, then lecture, then demonstration.
