Practice breath control until you can deliver whole verses without being out of breath or sounding too quiet or tired.
Make sure your rhymes are on time. Don't keep rhyming the same word unless it's on purpose. Stay on beat unless you are rhyming offbeat on purpose.
Use original rhymes, speaking to the occasion with immediate relevance. Make as many rhymes as you can using different parts of the word. Make the amount of rhymes that can flow out of you seem endless.
Punctuate your verses with punchlines. Lead up to a definite end to each phrase and deliver the punchline at the front of center stage. Squat down or swing an arm forward in a blow as you do it, or point at the audience. Read the book "The Science of Rap" by KRS-One, where this is described in detail.
When you make a claim in your verse, follow it up with a simile or metaphor. This adds credibility to your claims and shows the depth of your mind to the audience while drawing them closer to your point, or it repels and astounds them in graphic detail. The master of similes and metaphors in rap is Lord Finesse.
Speak of familiar subject matter so that you know what you are talking about and don't fumble on the mic. This builds rapport with the audience and makes you more relevant. If you are leading the audience into unfamiliar, abstract material, announce it and cue the people to follow along, but then don't make it too complicated. Let them actually follow you.
Memorize your material so you don't blank out while performing. Be prepared to make up new material on the spot, or freestyle. Don't be learn only one method.
Be confident. Confidence is attractive and infectious. It also blocks out anxiety and prevents you from freaking out if the sound system breaks or if you accidentally stuttered. You'll just breeze right through the bump or work the mistake into the rhyme.
Speak in a clear tone of voice and project. Change your tone and pitch to add variety as the song changes. Don't be monotone unless you are being monotone on purpose. Again, be deliberate.
Use a good unidirectional microphone such as a Shure and don't cup it with your hand. This will make your voice sound clear and punchy instead of muffled and garbled. If you are recording, then use software and effects to boost your voice into loud, clear stereo with some light reverb. Use AutoTune if you want to, but not if you have to. Don't record what you cannot perform later or your audience will be disappointed.