I wouldn't claim to be a great presenter, or even a good one, but here's a tip I thought of the other day.
For a better explanation of a "J-Cut" go to this article, but make sure you come back here OK?
Now you know how it applies to films, here's how it applies to presentations. Sometimes when I'm doing presentations I don't know what comes on the next slide, so I flick to it, get my brain around it and then start talking. It doesn't look good though. What is better is to have a printout of the slides, or know what is coming next. Then you can start talking about the next slide, and while you are talking, flip to it. I think its better practise anyway as the aim of the presentation is for people to listen to you. If the Powerpoint could work by itself, why are you needed? So make sure that when you change topic then you are talking, getting their attention, and when you are ready the image on the screen changes to illustrate what you are saying.
Of course you need to be able to control the slides. If you are in a situation where someone else is doing the transitions, as happens in our church they need to be pretty clued up on what you're saying so that they know when to switch.