It is not just my opinion.  There is a study on this.  I found this article during my normal bedtime reading. 

"The Influence of a Communicator's Body Position on Opinion Change in Others", Hugh McGinley, Richard LeFevre, and Pat McGinley, Journal ol Personality and Social Psychology, 197S, Vol. 31, No. 4, 686-690

Abstract:  Investigated whether, while discussing his or her views, a communicator who exhibits limb-outward or open body positions would effect greater opinion change in an addressee than a communicator who exhibits limb-inward or closed body positions. 96 female college undergraduates whose attitudes were premeasured perused an attitude questionnaire of a female student and then viewed pictures that were taken of her while she discussed her beliefs. Some Ss viewed open body position pictures of the communicator, while others viewed closed body position pictures.

Results:

Overall, the study showed significant difference between the open and closed groups, in terms of opinion change.  The other measures of 'Liking', and Potency, and Activity, are also significantly higher for the open body position.

The 'open' body positions used in the study include: combinations of the following: leaning backward, legs stretched out, knees apart, one ankle crossed over the other knee, elbows away from her body, hands held outward, and arms held outward from her body either directly at her side or elevated. Closed body positions include some sort of combination of the following: elbows next to her body, arms crossed, hands folded in her lap, knees pressed together, feet together, and legs crossed at either knees or the ankles.

Conclusion:  In order to get the audience's attention, to appear to be more active, project more power, then use a open body position and hand gesture.  Not surprisingly, you can be more persuasive in this position as well.