Purchase a rhyming dictionary from a bookstore. A rhyming dictionary contains a list of word, alongside words that rhyme. This is useful to use when writing lyrics.
Create a rhyme that incorporates simple objects in the environment. For instance, sit in a park and write a list of all objects -- bench, swing, sandbox, sneakers, clothing -- that you see. Create witty and artful lines incorporating those objects. For example, "while you play in the sand box, I rock the block..."
Write down any clever lyrics that you come up with and memorize them. When you need a lyric in the middle of a freestyle battle, you can pull from one that you've memorized. You should also write raps frequently. Writing raps will help your mind to get used to rhyming and coming up with clever lyrics.
List all current or popular events and celebrities. You can incorporate these events in your rhyme. For instance, after September 11th, many battle rappers incorporated this event into their rhyme. As time went on, they did not mention the tragedy as often, as it was no longer a recent event that topped the headlines.
Know your audience. A mature audience is often more aware of current news headlines or older celebrities. A young high school audience, however, might be more impressed with the mention of popular hip hop celebrities and may be unaware of many news events.
Stand in the mirror and practice your demeanor. You must appear confident and cocky. Your facial expressions must express your lyrics, or the lyrics will not come across as genuine to the audience. Rappers also use lots of hand gestures when performing.
Practice battling a rapper that is a better freestyler than you are. Take any pointers and constructive criticism offered by the more experienced rapper. Also, take mental notes of any lyrics and non-verbal gestures that make him a successful battle rapper.
Assess your opponent and organize your thoughts quickly. In a freestyle battle, you must quickly analyze your opponent and come up with a list of negatives: his shoes, his lyrics, his short stature or wide waistline. Organize these thoughts quickly, then start with an introductory line that will smoothly transition into your first lyric. End the battle with your strongest lyric.