first off i would say have good emotion when you are talking. believe what you are saying.
don't verbally bury your opponent. im not saying not to put yourself over or point out opponents flaws but you need to make them look like a threat or why would anyone what to watch you wrestle them. just make sure what your saying gets you over makes your opponent a threat and make sure to further the storyline.
also don't swear and try and use a few big or fancy words. by doing that you sound smarter and more sophisticated. plus when you do swear it will have way more power. just do it when its a really nessicary promo.
the ability to enunciate and not stammer is also a good quality.
being a little poetic also seems to go over well.
lastly a quick wit is very beneficial.