Flow is no great secret, despite what some may say. Singers have flow. Preachers have flow. With a little hard work, you can have flow.
It is, simply put, the vocal quality of your voice. I'd like to take the mystery out of flow in this article. Here's how to strengthen your flow:
MC's are essentially made up of two parts: flow and lyrics. In this exercise you only care about flow. Play a beat and freestyle over it, going as long as you can without stopping to breathe. Don't worry about rhyming; you can just as easily "doo whap" your way through the exercise. The focus should be on developing sounds that connect with each other and the beat.
Try rapping in different accents: English, French, Middle Eastern, whatever. You may laugh, but look at Slick Rick. Next time you freestyle with others, try to focus on lyrics. When you freestyle alone, focus on flow. When you are comfortable with both, try incorporating them together.
Lastly, you want to change your speed. Start off trying to rap as fast as possible, but with a constant beat (early Outkast albums are a lot like this). Then, slow it down and try and keep in time. Now, try to rhyme super fast for eight measures, then super slow for eight measures, then super fast for eight measures, and so on. By working out the extremes, you'll feel much more comfortable changing your speed while rapping, which will make you sound more original and polished.
Become a freestyle rap phenom with this free guide on How To Develop Rap Flow