I remember once hearing about Nate dogg and snoop dogg (lion) standing at school or somewhere and one of them would just point something out and the other would freestyle on it and they would just keep it going on and on whether it be about somebody's shoes or a car passing or anything just to get that quick eye to brain to mouth action. Get that standard no frills hip hop beat ingrained in your head or get a beat going and just start trying to write. One of the things I really like about rappers is the way the great ones interact with the beat and can weave in and out of it and fall behind and catch back up to it. Practice doing that with beats and using different rhythms With the same beat. Listen to some old ice cube and you can hear some of the beats feel kinda like they are lethargic and heavy but the way he rhymes over them give it this kinda smooth breezy feel. While you are doing this you don't have to really free style the lyrics you can freestyle the rhythms and let the words come to you as you practice.