Learn how to rap. You must know how to rap before you can rap fast. To rap, develop a feeling for beats and rhythm and combine it with lyrics.
Choose a song you want to rap. Ludacris, Lil Jon and T.I. have songs that are ideal for learning how to rap fast.
Practice saying tongue twisters over and over. This helps you become comfortable with talking fast and learning when to take breaths.
Become familiar with your song of choice. Listen to it as often as possible and memorize the lyrics.
Sing the rap slowly and alone, without the help of the song in the background. Do this over and over, singing it quicker and quicker each time. It helps to tape yourself singing it to know where you need improvement.