A few thoughts:

1. Improv classes (see discussion elsewhere in this thread)

2. Vocabulary - sounding "smart" doesn't necessarily mean "using lots of big words", but vocabulary does matter. If your speech is littered with too many "filler" words like "stuff" and other vague terms, you sound less informed than if you use more precise terminology. Having comprehensive domain knowledge in the field your discussing and knowing the vernacular, can help a lot.

3. I think you can learn a lot by listening to, or reading, speeches and essays by great orators and communicators. You can almost think of this as "modeling" in NLP (Neuro-Linguistic Programming, not Natural Language Processing) terms. If you think Obama, or Bill Clinton, or Martin Luther King, or Elon Musk, or Vladimir Putin, whoever, is a great communicator, search out and listen to and read their speeches. Winston Churchill is somebody interesting in this regard, because there's quite a bit of his stuff available online for free.

http://www.presidency.ucsb.edu/medialist.php?presid=42

https://archive.org/details/Winston_Churchill

4. "It's not what you say, it's how you say it." This quote is used a lot in the "pickup artist" scene, and there's a lot of truth to it. Delivery is crucial. This means tonality, volume, cadence, body language, everything. Something as simple as your posture effects how you communicate with other people. There is a lot of material out there on this, but a popular source is something called the "Alexander Technique". There are also a lot of books on body language. And there are voice coaches who can help fix quirks with your voice itself.

5. There's an old saying "the best way to learn to write well is to write a lot and read a lot" (paraphrased slightly). I think the same thing holds if you transform it to "the best way to learn to speak well is to speak a lot and read and listen a lot". Join Toastmasters (or find some other venue where you can speak in public) and start preparing and giving talks and speeches. Then turn around and consume as many talks and speeches as you can, and pay attention to the details of how the people who impress you speak. You can probably find some great TED talks and the like online to model from.

6. It sounds like an outdated idea, and it may offend the sensibilities of some reading this, but your physical appearance matters as well. If you look physically imposing, people have a tendency to be (at least slightly) more deferential and pay more attention to you. There was a book, I think by Bo Dietl, where the author made the point that "You always get more respect when it appears that you could kick the ass of anybody at the table". I'm not saying you need to get Arnold Schwarzenegger huge, but being physically fit has a lot of benefits. Lifting a few weights here and there might be a bad idea. Broad, strong shoulders, muscular arms, a thick neck, decent chest, etc., that give of an appearance of vitality and strength are probably good things to have.

7. Another thing to consider: If you find people have a tendency to cut you off and start talking over you, do not be afraid to make a little gesture (hold up a hand with your index finger, or index and middle fingers, pointing upward) and/or simply say "Hang on, I'm not finished yet" or "please let me finish making my point".

8. To revisit the NLP thing a moment... there's a LOT of material out there on using very specific speech patterns and linguistic constructs to help get your point across, effect other people's mental states, manage conversations, etc. There's book after book on this, but for starters, I'd look for "Sleight of Mouth" by Robert Dilts, "Reframing" by Richard Bandler, "The Persuasion Skills Black Book" by Rintu Basu, and "Pitching Anything" by Oren Klaff.

http://www.amazon.com/Sleight-Mouth-Robert-Dilts/dp/09169904...

http://www.amazon.com/Reframing-Neuro-Linguistic-Programming...

http://www.amazon.com/Persuasion-Skills-Black-Book-Practical...

http://www.amazon.com/Pitch-Anything-Innovative-Presenting-P...