You need to practice the webinars alone but in record mode. Using GoToMeeting/GoToWebinar you can practice presenting to yourself or others in your organization. The benefit is you get used to the technical tools to prevent those technical mismanagement issues as well as practicing your pitch. I also mean practice them all the way thru completely. With the proper tools you can and should record them so you can review yourself and/or distribute them as branding materials.