FREESTYLE RAP TIP #1: Write down your freestyle rap practice sessions.
I was driving around the city last week and threw on a mix tape of instrumentals. I started messing around freestyling when I suddenly came up with something like "...that didn't rhyme cuz I'm in the frame of mind, to simply fuckin say a line..."
Now I thought that was kinda funny so I kept a mental note of that line. So that the next time I don't rhyme a line in a freestyle rap, I can throw in that bar or a variation of it. And that's where writing down your best lines during your practice sessions come in handy.
At first just stick to listening to an assortment of instrumentals, slow, fast, everything, and keeping a notepad to jot down dope lines you may accidentally come up with...then just quickly forget them and keep freestyling. Then repeat the cycle. Over time you will have a nice thick notebook, move on to your next one.
FREESTYLE RAP TIP #2: Be patient, there's no need for perfection.
Don't feel obligated to use every single one of the lines your write down in your notebook. Those lines are just for reference. You should review your notebook every now and then to keep those lines fresh in your head if you ever need them and they fit in during a freestyle rap battle.
FREESTYLE RAP TIP #3: Become a master at observing your surroundings.
In a real freestyle battle, you can't just throw out an endless barrage of writtens (sometimes that may be acceptable in a radio station freestyle, but not a battle).
I remember a particular 106 & Park Freestyle Friday where one contestant who was battling spit a line against his opponent that went something like this:
"My sound scorches suckers when it comes out the speakers/ Everybody moment of silence: (pause) look at her sneakers."
That line got a roar out of the crowd because of many reasons, one being the tension that built up during the quick pause, another being the battler's keen observation that his opponent had on a pair of very worn out sneakers...