Know your subject
Structure what you want to say
Present the key points
Longer talks - tell them what you're going to tell them, tell them, tell them what you've told them
(i.e. set scene, talk, conclude)
Use examples to help people relate to the subject
Consider how to best get your point across - "chalk and talk" or "powerpoint and pontificate" are not always the most appropriate
Consider using discussion, questions, acting out scenarios and other approaches if they make the point well
Put the correct amount of material on each slide/page - not too much
Use pictures when appropriate
Don't get bogged down in artistic design
Don't use flashing stuff/unnecessary animations/pointless examples
Don't read the slides - talk around them
also, guide to writing essays - not directly relevant but some useful tips on research etc.