Basics
Talk to your audience rather than reading to them. You should use point form rather than written speeches. Eye contact matters, as done tone and pace. A more conversational tone is usually more persuasive.
Avoid vocalisms such as "um". and "Like" . Avoid repetitive terms, especially if they don't add anything to the case and waste time. For example many debaters often say "We tell you ...", before a number of reasons in their speeches, but the phrase is repetitive, takes up time to say, and actually weakens the case -- eg "we tell you it is going to increase the cost" is actually less convincing than "it is going to increase the coast" since the first sounds like opinion while the second sounds like fact.
Vocabulary
try not to be too casual or colloquial - more formal language can add to your persuasiveness e.g. avoid words like "sucks". "screwed"
avoid offensive terms. Err on the side of caution
Avoid mush words - e.g. stuff, like, sort of, kinda, I guess
Do not sound too formal or artificial.
"Irregular" verbs and emotive language:
use the right ones for your side and rephrase theirs
pay attention to the sound of the words
think about the connotation of words
avoid overused phrases or clichés
Vary your pace and tone. A more conversational tone is usually more persuasive.
Changing poor habits
words, phrases, or gestures repeated too frequently or regularly can become distractions
ask your partner to look for them because often you won't notice them
videorecording can also help you to see them