I try to be as flexible as possible with what I attempt to cover in one lecture. If the students seem interested in certain aspects of the lecture, or particularly confused with a concept, I spend more time on it than I planned. If I feel there is no flexibility it usually means I am trying to get through too much information. Obviously there are some limits to the flexibility. For example, often there is information that the students need to be given in lectures for a corresponding practical to make sense.
Enthusiasm is the key to a good presentation.
It s hard not to get excited about something when the person giving the presentation is passionate about what they are saying. This is just my approach to selecting and presenting information and I am sure there are many others. However, if I were a betting man and was asked to bet on the one thing common to all the different approaches that helped students to learn, my money would be on enthusiasm. Responses advancing the scholarly debate of issues raised will be published in the Web version of that issue.