Lyrics are the most important weapon in a rapper's arsenal. Read as much as you can; literature, periodicals, non-fiction, history and current events are all fodder for lyrical content. An MC who is well read and well versed in a wide array of topics will benefit from not only a large, diverse vocabulary, but will also have large amounts of material from which to draw. Practice writing. Taking creative writing and poetry classes also helps in this area. A rapper is judged by his lyrics, so anything you can do to make yours stand out will be helpful.
Rapping is a physical act, and breath control is an often-overlooked but extremely necessary skill for the rapper, especially in a live setting. Breath control is the ability to maintain an even cadence while delivering verses without running out of breath. Cardiovascular exercises are excellent for increasing breath control. Try jogging or general gym work (boxing exercises, rope-jumping, etc.) while reciting verses to increase stamina and vocal cord strength.
Being a better rapper takes more than just a rhyming dictionary. It is important to study and learn what goes into constructing a good song. Depending on the type of rap you're attracted to, there are many aspects of traditional songwriting that can help transform a bunch of words into a full-fledged song. Hooks, choruses and call-and-response chants are popular tools for creating rap music. Develop a good listening ear by studying artists who are known for their craft. Learn how to tailor your lyrics to the beat over which you are writing.
Remember, the essential foundation of rap music is an equal combination of beats and rhymes. Whether you produce your own beats and tracks or work with various producers, work to your strengths. For example, if your lyrical content is primarily gritty reality-based rhymes, you may want to work over tracks that have a more rugged, hardcore vibe to them. Conversely, if your lyrics are more into the poetic, abstract side, consider smoother jazz-influenced tracks. As you progress, you should develop a distinct style of your own, and the beats you rhyme over will play a big part in that.
Most importantly, practice as much as possible. Keep a journal to keep your writing skills current. Write as much as you can. Find places to hone your craft: open-mike nights, battle competitions, etc. Anything that encourages the practice of freestyle rapping will help develop flow and timing and will keep your MC skills sharp.