My buddy Dan Vega recently hit me up Twitter about how to give online presentations. Not that I think I'm an expert in the field, I do have some experience with this type of presentation and I thought I'd share some general tips. Here they are - in no particular order. I definitely hope some of my readers chime in here with some comments on their suggestions.
In General #
First and foremost - an online presentation (or "webinar") can be an extremely convenient way to reach a lot of people at once. As a speaker, you don't have to travel. As an attendee, you don't have to travel. Because of this, I can say that online presentations are the way that I've reached the most people in my career. I've given webinars to nearly 500 people - and heck - I didn't even know it till later when the organizer shared the stats with me. All in all, the ease of attendance is a great benefit, but there are some negatives as well.
As a speaker, I rely strongly on how my audience is reacting. If I see people fidgeting, checking social networks, etc, then I know I'm boring them. If I see a lot of confusion, then I know I'm not explaining things well. You get none of that when presenting online and it can be very offputting for the speaker. Because of this, I typically scale back on the jokes and funny pictures. I don't cut them all out, but if I know a particular bit of humor relies on me saying something fast, slow, sarcastically, or whatever, or with a crazed look on my face, then I'll cut it out. In general, I give very casual (heck, bordering on unprofessional maybe) presentations, but for webinars I'm just a bit more serious. Again - without the audience being able to see my face, I feel like I need to tone down my personality a bit.
On the flip side, you may not always have the most attentive audience either. There isn't much you can do about that, outside of trying really hard to give a great presentation, but I can say that I've attended webinars in the past where five minutes in, it was in the background and I was doing something else.
Another issue for the audience, and the speaker, is that Q&A will not be as easy as in person presentation. Different webinar platforms have different ways of handling this. If you're lucky, you'll have a system that separates questions from general chatter. (Oh, huge tip - ignore the chat. Trust me - ignore it.) If you're even more lucky, you'll have someone who can help filter questions and remove things don't belong. In every webinar I've given with a dedicated question system, someone has put comments there and "+1"s.
In general, if you are used to stopping for questions at various points within the presentation, you should instead plan on addressing questions at the end. I will typically warn people about this. I also try to ensure folks see my contact information in case they have to leave early, or if the webinar system doesn't have a nice Q&A system.
The Software #
Most times, I do not have a choice in what software I use to give the presentation. You want to make sure you test it out beforehand. Be aware of how you switch to screen sharing and how to turn it off. Many webinar tools have the ability to show slides and do screen share, but for me, since most of my presentations are "slides + demos", I will not use this feature. Instead I screen share and do my slides there instead.
If the system supports broadcasting your audio over the Internet, use that. It seems like organizers always prefer you to dial in. Frankly I think that is silly. Even in the best of situations, a call can drop. Or heck, you get call waiting coming in which can be distracting. If a system doesn't support VOIP, then I'm almost consider that a red flag for the overall quality of the system as whole.
Also watch out for Flash and Java-based systems. I promise you that as soon as you begin presenting, you'll get some bullcrap warning about either of them being out of date. Be prepared for it and don't let it throw you. You definitely don't want to hit the wrong button and fire off an update right as you start your presentation.
I used to love Adobe Connect, but the last few times I've used it, it stopped broadcasting audio about twenty minutes in. I avoid it like the plague now. I think it is run by the same group doing ColdFusion, so there ya go. (Sorry, that was bitter and I should delete it, but I won't.)
When I get to pick, I use Google Hangouts on Air. It has all the features you expect (video, audio, screen sharing, even various other plugins), and has the extra benefit of saving your presentation to YouTube when done. You can see an example of this in my last webinar.
Another tip - don't forget to lower your resolution. Normally when you present in person, the hardware itself will drop you down quite a bit. This is actually a good thing as it will make your slides and code a bit more readable. Many webinar tools will not drop your resolution. Do it beforehand so you're ready to go.
The Rest #
It goes without saying that all the advice you've gotten for "regular" presentations apply online as well. Maybe even more so since you're so disconnected from your audience. A large part of what makes a presentation enjoyable and educational is the personality of the speaker. You'll have to work extra hard to allow your personality to shine through.
Oh - and don't think that because you're using the Internet you can just give your presentation anywhere. I've heard horror stories of presenters doing webinars in Starbucks. Just because you can do something doesn't mean you should do something. If you are giving one from home, be sure you have a quiet place. Kick out the pets. (I've had cats jump up on my keyboard in the middle of a webinar.) Kick out the kids. I let my spouse know so she can head off any loud child or other issue. Have a ceiling fan? Turn it off if it is too loud. Does your AC create a loud drone? Turn it off. (Yes, sweat a bit, no one can see you.)
I hope this helps - and I know I'm leaving out quite a bit. Leave a comment below with your own suggestions!