When you are moving on the stage, make sure that your movement has a purpose.  If you take a step, go at least three steps in that direction to cue the audience that you are moving for a reason.  Then, stop and finish making your point before you leave that spot. 

One of the biggest problems speakers make is that many of them wander around or take a step here and a step there.  This is extremely distracting to the audience.  This doesn't mean that every step has to match a place in your speech.  Just tie it to those points you want to emphasize.

When making an important point during a speaking engagement, move toward the audience.  Three steps forward from center stage would be a very powerful position that would command attention.

Upstage (away from the audience) left and right are weak positions (especially left).  They can be used when you feel you are overpowering the audience or when you want to remove attention from yourself.  If you are comparing your idea to another, and you want yours to have more power, use this position for the opposing idea. This makes the other idea seem to have less power compared to yours.

Upstage center is a strong position, but one that makes you appear disconnected from the audience. You should try avoid this position.

If you want to really get the audience involved, go right into the crowd.  You may even want to address one or two people personally. Be sure to stop before you get to their personal space (less than 2 feet away).  Good public speakers get really connected to their audience. Create interaction with them and they will love it!

The main thing you have to watch out for when you are out in the audience is that in large rooms with lots of attendees many people can't see you, so they start to lose interest if you stay out there too long.