I think the most important thing is for everyone to know the purpose of the meeting: what's going to be discussed, what decisions will be made, etc. Without this information, participants have no way of preparing for the meeting, and productivity suffers. The vast majority of meetings I'm asked to attend don't provide this basic information.
Another good practice is distributing a summary of decisions and action items after the meeting.
If you're the person running the meeting, it's your responsibility to make sure that everyone is heard, not just the loud and assertive people.
