Controlling breathing is the most important aspect of vocal delivery. Regardless of style or lyrics, an emcee must be able to deliver his lines with strength and conviction. Nothing obscures a great lyric like a rapper running out of breath before he can finish a bar. There are many vocal exercises that can help increase strength and stamina. Deep-breathing exercises are most effective. One trick is to recite lyrics while jogging or running.
As a rapper, your lyrics are your most powerful tool. But what good is a clever rhyme if the audience can't understand it? Practice through repetition is a good way to improve your technique. Working on cadence and word emphasis helps the rapper bring his point home clearly and effectively.
While recording in a studio is a different dynamic than performing live, the basics are still the same. Good equipment is key in recording, as is an engineer that understands what the artist is going for and how best to get that sound recorded. While in a vocal booth, mic techniques are the same: keep the levels even. If the artist cannot hear himself well there is a tendency to shout and distort the vocals. If the sound in the headphones is too high, the artist may record at a hushed level. Again, breath control and diction come into play.
A strong rapper knows how to tailor his delivery to the beat. A song that is constructed over a gritty, hardcore beat requires different technique than one based on subtle instrumentation. The rapper's energy level is key here: too much can sometimes make a song unlikeable and too little makes it dull. As an example, a song with a reggae or dancehall feel will require a different vocal delivery than one that contains elements of funk or R&B.