Since I graduated from High School, I've had the chance to attend a lot of presentations (talks, conferences, lectures), given either by students to teachers, or by some non-student people. Most of them are tech people. I would like to distinguish two kinds of presentation:

Those given by students or employees, often at the end of projects, or to explain quite serious things, or to expose problems. I will call them “formal presentations”
Those given by any kind of people, aiming at making the attendees discover technologies, languages, tools. I like to refers to these lecturers as evangelists. I will now call these presentations “informal presentations”, since they rarely happen in a professional or university context.

Now let's get started with some pieces of advice. Then, you can go even further by reading this great article about 10 presentation anti-patterns.

Know your topic

This may sound obvious and yet, some people (they're pretty rare) don't know perfectly their topic. They hesitate, or even worse, say absolute nonsense. Prepare your topic if you're not very familiar with it, but know it.

Know your speech (or pitch)

Knowing your topic is not enough. You will often be granted an allocated time. To avoid exceeding that time, rehearse a lot before going on stage. No secret, just repeat until you feel confident with your time management.

Smile

This is really true for “informal presentations”, but it also applies to “formal presentations” (but don't overdo it). Show your audience you're happy to be there. Then, it will help you deliver your message. Set aside your problems. No sound on the computer? It doesn't matter, apologize and keep on smiling. The demo failed? It's OK. Do not show signs of stress. Crack a joke about it (if appropriate, depending on the audience).

Filming?

Add the slides on the righthand side, inside the video. It's a terrible thing for those who will watch you online, if you make a joke about what is on the slides and they cannot actually see them.

Also, when asked a question, make sure that the speaker repeats the question in the microphone, so that the web audience can understand the answer. Another alternative is to use two microphones, but this is not very handy, since someone has to move the microphone around everytime a question is asked.

Look at the audience

Attendees can easily get distracted. Establish a privileged contact with them. Don't look at your slides. I know it's really hard not to, but try to find a way to get a display in front of you that reminds you of the slide you are on. People will give you credit. On top of that, you would also benefit from speaking slowly, at an understandable pace so that you don't lose anyone in your thoughts. Allow people to concentrate and understand what you just said: mark pauses between slides, when need be.

The Art of Creating Slides

The content

A 140-character-long slide is OK, but not more! Use simple words (often keywords) and images or videos (as backgrounds). Funny slides help a lot, people won't get bored.

Avoid PowerPoint and LibreOffice Impress

You have no guarantee that the version of PowerPoint (or LibreOffice) you'll find on the computer is the same as yours. What's more, not everyone can afford Microsoft Office suite. The “source code” of a slideshow should be a text file, and the output should be a PDF file. Stick to this rule and everything's gonna be fine!

Giving a “formal presentation”? Use LaTeX to generate the PDF. Otherwise, use Markdown to generate your PDF. There are a lot of tools for that. Personally, on Linux, I recommend Pandoc.

Why LaTeX? The formatting looks very professional. Just like scientific reports are always writtin with LaTeX, your slides should be as well.

Why Markdown for your “informal presentations”? It allows you to create your slides in only a couple of minutes. Don't spend too much time on transitions, nor on the colors, etc. The only things that matters is the content, and Markdown is all about content. You can even get some help from my own slides.

Next slide, please

Don't rely on anyone but yourself to switch between your slides! Whenever possible, use a cheap remote controller (a presenter).

Holding the microphone the right way

Place the microphone on your chin. If you have a beard, put your thumb between your chin and the microphone to avoid rustling noises. That's the only way of keeping a constant and good sound level.

I know these pieces of advice can sometimes be hard to follow, I also made and still make a lot of mistakes, but I am constantly trying to improve myself. Watch your past presentations, notice what you did wrong and keep on improving yourself. Good luck!

How To Give A Good Presentation