I find that pausing - or what they call "dead air time" is the biggest killer. Keep talking, keep it going as smoothly as possible and that tends to keep people interested. If you are at least knowledgeable on your chosen game and can convey that across confidently, you'll keep people watching. Or if you're honest and admit you don't know but through talking to the audience you show you're learning the game, they watch and learn with you.