The most important advice I've ever been given is to time myself talking through my slides. It is incredibly difficult to guess how long a particular presentation will take, which makes things rather awkward when you have conference organizers with schedules; talking through your slides gives me an accurate number which can result in me adding or removing slides. Even when I don't change the slides, knowing if I have plenty of time or need to squeeze helps me determine how much to re-iterate points and let things "sink in" before moving on to the next point.