Send this URL out on your social media networks with the date, time and time zone of your live chat and encourage your fans to send their questions to a specific twitter account / hashtag in advance of the chat.
Make sure to blast your email email lists and social network followers many times leading up to the event and get them to show up.
Test the internet connection at http://www.speedtest.net at your live chat location a week in advance to make sure you have sufficient upload speed. You want to have at least 1.5Mbps upload or greater. Anything less might mean that you'll have to settle for low quality, your video will get choppy or at worst, the broadcast might go off air. Favor wired internet connections to wireless and try to limit other people using the same network at the time you are live.
Get a Logitech C920 HD USB camera and use the newest, most powerful laptop you can find.
Download Ustream Producer http://www.ustream.tv/producer and do a test broadcast a week in advance. Watch the test broadcast and make sure you can hear and see everything clearly and do a trial run of the chat. Be sure to troubleshoot and resolve any technical issues well in advance of your live time.
Make sure you do another speedtest before broadcasting and another test broadcast. You can do your tests to a separate channel (you can create additional channels in your account for free) if you don't want people seeing your testing. Make sure your internet connection and your video and audio are good.
Start your broadcast early, but start your chat late. Most of the audience won't show up on time. Start the stream 10 minutes before your advertised live time and have your talent not show up until 10 minutes after the advertised live time.
Have some filler material like a trailer clip or a graphic with some messaging so people know to wait and build the anticipation. In Ustream Producer, you can add music, images and video clips easily. In this pre-show messaging encourage people to keep sending questions to the dedicated twitter @username or #hashtag or to an email address, website, or however you are gathering questions.
Be prepared with a "monologue" or "rant" to start off your live chat. People might not show up or start asking questions right away. Start strong in a high-energy fashion with a message about your latest news / album / project / adventures / juicy gossip.
Let your talent read the Ustream social stream directly and don't filter questions for them. Let the fans see that they are reading their messages directly and put them close to the camera so that they have an intimate one-on-one style chat with the talent.