Some good answers here already. Mostly it comes down to practice. If you know your material and know that your presentation is interesting and engaging, you'll feel a lot more confident.
I teach public speaking classes at General Assembly. Couple of quick tips.
1 - Always open with a joke, story or question. It helps build rapport with your audience (whether it's a live presentation, a webinar or a recorded video). Of the three a joke is best, provided it's actually funny and relevant.
2 - Watch other presentations/webinars/teleseminars. Find ones you think are good, then figure out WHY you think they're good. Pick out words and phrases that you like and use them in your own webinars.
3 - Practice. do lots of webinars. Try doing a few public speaking classes. Volunteer to speak at events or debates - the more time you have getting your point across to an audience, the better you'll get.