Practical Advice for Remote Delivery of Lectures

Sometimes you've just got to deliver some lecture content and you can't be there in person (illness or transport mishaps might mitigate against proper face-to-face engagement). You have some material (perhaps some powerpoints and some web pages), and you would like to carry on "as normal" (with question and answers and student interaction).

What's the best, practical way to handle these situations in a bog standard lecture theatre where specialist e-learning tech isn't to hand? A mobile phone + a microphone + a teaching assistant to advance the powerpoints? Skype + video + powerpoints shown through a shared desktop? How much planning and checking beforehand? What's the best you have achieved? What tends to go wrong?

(I speak from the perspective of having organised such sessions for lecturers with broken legs or sick relatives. I have also been on the receiving end when I lost my passport and couldn't travel to give a tutorial at a conference.)