Les Brown da gawd
a couple have been said already: read(very important) and listen to great speakers on youtube.
a few more:
expand your vocabulary: don't just say big words for the sake of it, use them properly. This mainly comes from reading so two birds with one stone.
Say more with less: a lot of people I've done business with or interacted with tend to fail at this, takes them 10 minutes to say something that should take them 5 minutes to say.
and if it's 1 on 1 speaking that you're wanting to improve then learn active listening. I often get told that I am a great listener and people love talking to me because of this. Acknowledge what people are saying. I used to sound like a Young Jeezy adlib before I learned active listening ("Yeah", "That's right"). I even ask questions about specific things about what people are saying (works great with meeting ladies too), for example "Oh, you say went to school at {school} and you're studying {major}, that's cool, how are you liking the school? What's your major about?"
And kinda left field and unpopular opinion, but read some good copywriting blogs, copywriters have a way with words because their words are money (almost literally), google the Gary Halbert Letters, the guy has a good command on lanuage and has made millions writing those sales letters.
Oh and if you're up to it, take a public speaking course, shyt is scary at first. I took that class over 10 years ago in community college and it has gone a long way and has improved every aspect of my life.
Good luck breh.