Get in to the music. Words and music have to be a happy marriage not a messy blind date. Don't try to force words onto the music. If yo have written something down, you can keep the main idea and a lot of the words. But don't rigidly stick to what's on the paper. The music and words have to flow together.
Keep your mind focused on songwriting. The more you keep your mind focused on writing, the better your content is going to be. Things will strike you about life as you go about your everyday business.
Prepare to start writing anywhere the inspiration strikes you. Write on napkins, newspapers, toilet paper or your hand. If you've got the idea, grab it and keep it.
Jump on your impulses. Regardless of what you have written down on the paper, you've got to go with the flow of the music. Don't be afraid to go in a different direction than you intended.
Expand your vocabulary. A lot of music, hip hop and otherwise, relies too much on generic and vague words. If you are not saying exactly what you mean, then the listeners aren't going to connect with it. In this case there's really no point in you even writing. Be as specific as you can. It's much more rewarding.
Rhyme only when it hits you. Rhyming is a great convention in hip hop music, but it shouldn't restrict your message. There are a lot of corny rhymes out there because people won't let go of the traditional rhyme scheme. Try sounding cool and rhyming something with the word "orange." It's impossible.