Keep a notebook with you at all times. Write down any rhymes you can think of, whenever they come to you. It may be a line or two, or even just a couple of words, but accruing a notebook of rhymes will help you formulate songs later.
Study other rappers you want to emulate. While it's important to create your own style, you also need a jumping-off point. It's also important that you enjoy the rapper's music; don't listen to someone because they sell a lot of records. If you like Afrika Bambaataa, and you dislike Kanye West, don't emulate Kanye just so you can sell records. You have to stay true to yourself. Being real is an important part of hip-hop.
Practice exercises to help you enunciate better. Rapping requires you to speak quickly and rhyme often, which can cause words to become jumbled. Since lyrics are such an essential part of rapping, they must be heard clearly in order for the song to be good.
Obtain some instrumental music to practicing rapping over. If you're very skilled at creating music, then create your own, but otherwise, leave the beat-making up to professionals. Getting beats from an outside party also allows you to focus on your main skill instead of having to worry about making music. Listen to the beat and focus on rapping in between it. You want your words to interweave with the drums and bass line, as opposed to having them all occur at the exact same time.
Memorize your lyrics before you record. Reading them off of your notebook in front of the mic can cause a lack of emotion. By having them in your head, you can focus on their meaning while you speak, as opposed to trying to comprehend them and emote simultaneously.
Refrain from drinking milk or eating other dairy products before you perform or record. Dairy products cause your body to overproduce mucus. This ultimately results in you clearing your throat more often, which irritates the vocal cords, and can make your rapping sound worse. Drink water often, especially before performances or recording sessions.
Be confident. Commit to your lyrics and flow whenever you perform or record. Speak loudly, clearly and proudly. Believe in yourself and put forth an energy that you're proud to represent.