1.Watch online battles you can find them on Young Rapper Spot. They are a good representation of what happens in a freestyle battle.
2.Listen to diss raps by artists on other artists. I reccomend you listen to Girls by Eminem. Here's the link : http://youngrapperspot.blogspot.com/2011/01/eminem-girls-lyrics.html
3.Pay careful attention to artists and try to mirror the techniques they use in a battle. This way you will not only learn more tricks but also enhance on your own methods.
4.Write down some punchlines. There are very few Rappers who can battle with any pre-written lines and the best example is PROOF! Just incase you do not know what to write or how? check this page out : WRlTE
5.Keep practicing. Use any spare time to practice freestyle. Do not stop even when you run out of words, you need to keep the flow going. The more you practice the more stamina you have and the better will be your result. Its kinda like a mental work out.
6.Visualize your opponent. Take a picture of someone you hate or look at your dog and try to visualise that person you want to diss and you will soon come up with smart ways to take down your opponent.
7.When you diss be sure that it is true. You should not consider cooking up stories because the crowd hates liars.
8.Never mind if you lose your initial battles. Each time you are knocked down you stand up even stronger. Dont think you have lost it instead prepare for the next time.
9.Do not get nervous and never ever feel bad when its your opponents turn. Stay focused and be confidence.
10.Include as much as humor you can. Make the audience laugh that's when they like it.