Call from a quiet location.
Don't put your phone on hold if you have on-hold music or advertisements, as this will play back into the conference room.
Consider keeping your phone on mute (by pressing *6) to keep out extraneous background noise.
You'll want to have as strong a connection to the conference as possible. For example, a landline will sound better than a cell phone or VoIP line.
Make sure that you have all the correct dial-in information in advance of your call.
Try to arrive at the designated conference start time, so that others aren't waiting for you before beginning the session.
Announce yourself as you join the conference call, as a courtesy to the rest of the participants.
Consider recording the conference call for those who are unable to attend.
Request that your participants review any meeting notes or comments prior to joining the conference.
Always speak clearly, and vary the tone of your voice to express yourself accordingly.
If it's your first time using the service, consider conducting a test session to familiarize yourself with the touch-tone commands and other functionality.
Disable your phone's Call Waiting (by dialing *70 before the conference number), otherwise the beeping may disrupt the call.