Here are some top tips for improving your speaking and presenting skills. These apply just as much to online talks as to in-person events, and all of this will get easier with practice - don't worry if you forget or find it hard at first.

1. Speak slowly and clearly – this isn’t easy, but you’ll be easier to understand, and will seem calmer and more confident. Aim to speak slightly slower than your usual talking speed. Tone and pace can be effective tools in setting the mood and bringing things to life.
2. More for real-life talks, but: stand still if you can. Walking around while presenting, or rocking back and forth, is common, but it can be distracting. Plant your feet and claim the space you’re standing in. Comedians often use movement to direct audiences as to when they should be listening – standing still to deliver a joke, then walking around while the audience laugh – then stopping when they want people’s attention back.
3. Face the audience/camera as much as possible, and don’t cover your face – it’ll make it easier to hear you, and if people can see your face, they’ll be able to lip-read. (Wearing lipstick makes it easier for people to lip-read on a webcam.) Don’t look behind you at your slides and carry on talking! For in-person talks, presenter view on the computer that’s projecting can allow you to see the slide, and what’s coming next, on a screen in front of you (as well as useful timers and presenter notes). There are ways to set this up for online talks too.
4. Eye contact (or, equivalently, looking straight at the camera) is important to show people you’re talking to them. In person, look at each member of the audience an equal amount of the time. On a webcam, try sticking a picture of a smiley face near the camera, or moving the window you’re looking at to the top middle of the screen, so your eye-line is where the camera is.
5. Don’t have too rigid a script. While it’s good to have certain key phrases and punchlines delivered in the same way every time, you can work from bullet points for the most part, and you’ll come across more naturally. You know the material! Just tell us about it.
6. Don’t do what you know is rubbish. You will have seen and know what makes a bad presentation. Make a list of those things – then don’t do them! Watch other people doing talks/workshops – attend events or watch videos on YouTube. Not all of it will be perfect, but think about your experience from the audience side – what works well, and what doesn’t?
7. Practice is key. Find opportunities to try things in an environment where it’s less crucial first; explain things to friends. Practise on someone who’s not a scientist. Recording yourself giving a session and watching it back will be, frankly, awful, but will help you improve a lot. Ask colleagues to sit in your sessions and be prepared to take honest feedback - and don’t take it personally. (Unsolicited feedback can be annoying, so ask for it if you want some!)
8. Don’t use jargon. You’re not there to impress people but to help them understand - using jargon can sometimes be a defense mechanism. You don’t need to use it to defend your status as an expert; it just makes you look like you don’t understand your audience. It is usable if introduced correctly, and if participants come away knowing a new word, that’s good. Also, watch out for normal words which have a different meaning in the context, and make it clear what you mean.
9. Hooks: this can include exciting moments, jokes, teasers, demos, etc. Good effective hooks can help with making memories, which helps new knowledge stick. The principle of Catch-Hold-Reveal is used in magic to structure a trick, but can be applied to demos, explanations or activities.
10. Scaffolding: build the things people will need to know first; order is important. However, don’t labour things too much or add in detail that’s unnecessary - when explaining for TV, the presenter often has to remove the ‘pyramid of knowledge’ - and you’d be surprised how much you can take out.

You will almost never come away from presenting feeling like you've done everything you could perfectly: there'll always be a way to improve, but you should interpret that as a positive thing and make notes or lists of things you've learned, so you can do better in future.