Rule number one of Google Hangout: Anything that could go wrong, will.
That said, here are a few things you can do to make it better.
- Get on it 15 minutes before event time to get over technical difficulties.
- Go live 5 minutes before event time so people who come early can already see the live stream. Otherwise their window will stay static until they refresh i.e. they won't know the event has started. Also, by starting a bit early people can start posting questions already.
- Send reminder emails / tweet after you go live for the same reason as number 2.
- If you use the Q&A module, beware that it may or may not work. So you may need a different place to post questions e.g. the event comments.
- It's better to have another person to keep an eye on the questions so you can focus on talking. Have that person ask the questions for you to answer.
Here is an example of a Google Hangout on Air we did for 360|AnDev: