Checks before the show starts.
Check the Internet connection and Skype by calling "Skype Test Call" lady. Ensure that the voice is clear on both ends of the conversation.
Start Audacity (if you plan to record the conversation) and set the recording to an appropriate source to record both ends of the conversation. On some computers it could be "Wave Out Mix", while others may have to use "What You Hear". Start recording and call the Skype lady again. After the test, ensure that both ends were recorded with appropriate sound levels.
Ensure that the streaming server (Shoutcast or any other server that will be used) is available, and you have access priviliges for streaming.
If other people are going to join in for an interview or discussion, request them to check their mic and speakers.
Make a list of all important points that will be covered in the talk.
If the talk is schedule bound then having a software timer set with little alarms or screen flashes will help keeping up the schedule.