Learn how to rap effectively. Beyond good lyrics and attention to rhythm, there are a few techniques you can use to be better understood and get your message across.
Stress the consonants. If you try to rap the way you talk, it won't be comprehensible.
Keep your words clear. Pay attention to keeping your words sharp.
Rhythm is more important than rhyme. Don't stumble or stop if your freestyle doesn't rhyme—just stay on-beat and everything will be fine.
Be able to think about your next line while still giving 100% to the line you're currently rapping.
Be loud! While being too loud is never a good thing, it's important to both literally and metaphorically be heard.