Follow LL Cool J's Advice. One of the greatest rappers of all time, LL once recorded a track where he rapped the alphabet while saying "the" in between each letter.
So it came out like "The A-The B-The C- The D, etc."
This is harder than it sounds, and is a great exercise to help you rap fast. Try going through the alphabet, saying each letter as clearly as possible, and increase the speed at which you rap it. After a week or so you will start to see results.