The short answer to the title question is, of course, don't hold one at all.

The long answer is:

A) Don't let the first 18 minutes of your 60 minute time slot go by futzing with technical difficulties, letting your audience listen to your team's "uh, why don't we try x?" and "Bob, can you see the whole slide now?" If you have 100-200 people on the line, you've just wasted close to one human work week of their collective time. Get on the line an hour early and do a dry run!

B) Don't announce that you're going to make a brief pitch for your company's services, then proceed to deliver an infomercial consisting of little else. Especially don't answer audience questions with sales pitches. "Well Bob, the XYZ certification may be the industry standard, but doesn't that mean that everybody has it? If you really want to stand out in the crowd, we recommend also getting the one we sell."

C) Especially don't combine A) and B). Why on earth would I sign up and pay tens of hundreds of dollars to take a training course from someone who can't mount a basic webinar consisting of slide sharing and voiceover? Why, oh why?

Just had to vent. If Agile Learning Labs ever holds a stupendously bad webinar, I hereby promise to eat my shoe.