A typical mistake during many conferences is to barricade the speaker on the stage which creates an obstacle to the creation of a direct connection between the speaker and his audience. Any physical object that obstacles the line of sight between them is reducing the capacity of the presenter to communicate, specially his non-verbal language.
Barriers can be physical (desks, podiums, chairs, etc), environmental (insufficient illumination, room temperature, etc) or psychological (produced by elements that prevent a connection between speaker and audience). In this article I concentrate mainly on physical obstacles.
If the stage is crowded with furniture like desks, podiums or chairs -in most cases- they will be used!
A desk, specially if big or massive, represents a clear separation between the speaker and the audience. Besides, if the guy on the stage is sitting down during his presentation, his body language is severely crippled, thus reducing his communication skills (it becomes a psychological obstacle). An exception should be considered when the speaker actively needs a desk for his presentation.
For example Edward De Bono accompanies his speeches by drawing on translucent slides that are projected on the screens, so he needs the table to be able to draw.
Same thing applies to having chairs on stage. If the speaker sits down, his movement around and up&down the stage will be reduced.
An exception should be considered when the speaker has physical problems that prevent him from standing up on stage (because of his age, physical condition, etc) and when you have a discussion between 3 or more people. When 2 people are on stage simultaneously and they have to present together or discuss on an argument, I suggest to have them both standing up. It helps to create better dynamics between speakers. If you do have to have chairs, make them stylish and comfortable.
It might happen that when a speaker is too short he might prefer to sit down because in that way his height is less evident, so provide chair or desks if requested. He will feel more confident and his presentation will be surely better.
Lecterns or podiums are also a physical barrier, specially when they are positioned on one of the sides of the stage. From one part, the speaker will be near just to part of the audience. At the same time, he will be hiding most of his body behind it. A podium is useful for holding a computer that has to be manually operated during the speech (why not use a remote?) or some other stuff that the speaker will grab while he is speaking. If it will be used during all the presentation, then consider positioning it in the center of the stage. If it will serve only as an occasional support, then move it to one of the sides and have the speaker walk towards it when necessary (in this case it can be replaced by a table). Politicians are particularly fond of using podiums, specially because many times it has an attached screen that scrolls the text of the speech.
Most TED conferences have an empty stage, which facilitates communication and interaction.
Help the speaker walk through the audience
Add a central staircase to the stage and create big corridors between groups of spectators so that the speaker can come down and interact or just move around between people of the audience. Make sure that the wireless tools used by the speaker (microphone and eventually the remote controller used to manage multimedia presentations) have enough range and work ok even if the person is moving around the room. If there are limits to his mobility, clearly show the speaker the maximum reach. It sometimes helps to signal the limits of the wireless range by adding tape on the floor. Tom Peters is the kind of person that loves walking around the room while presenting.