Former tech trainer and presenter here. In no particular order:

* start by telling everyone what you're going to tell them, then tell them, then tell them what you just told them;

* know your material well enough that when your computer stops working you can carry on seamlessly;

* rehearse in front of a mirror or record and watch yourself;

* if you're talking too quickly, take a quiet breath and make sure you pronounce the last letter of each word that you speak;

* switch your slide to black or white when you're making a very important point, and have them focus on you instead of the screen -- use `b` key for black and `w` key for white, works with most presentational software I've used;

* …and smile