Write what you know. If you know about life as a 28-year-old white boy from Minnesota, write about it. If you are into hod rods and fixing cars, think about automobile-related metaphors and put them in your poetry. If you're crazy about the smell of a spring breeze and the sound of a river flowing, put that in your lyrics. Yes, all of these things can be cool--as long as you know about them and write from the heart.
Write for music. Get a feel for the basic 4-4 rhythm scheme of hip hop. Understand how the bass drum and snare drum create downbeats and upbeats and tailor your words to weave in and out of this danceable structure.
Vary your rhymes and rhythms. There is nothing more boring than a completely predictable rhythm and rhyme scheme. Try placing rhymes in the middle of lines. Place ending syllables earlier or later in the musical measure. Oscillate between single time and double time once in awhile to shake things up. Keep things interesting by using polyrhythms--falling in and out of the beat for an organic effect. Try switching from couplets to quadruplets, or extending or reducing the length of time between rhymes.
Practice with beats. Use your favorite DJ's songs as a background when writing hip hop lyrics. Try to match the feel of the music by writing poetry that compliments, matches, or contrasts with the mood of the music.
Read. Listen to hip hop lyricists whom you admire. Expand your vocabulary. The more experience you have as a consumer of hip hop lyrics, the better a producer you will become.
Get to know the basic structure of popular hip hop songs. Songs usually open with an introductory passage, move into a verse, state the chorus, do another verse, repeat the chorus, move onto a bridge or break, come back in with another verse, then close with another chorus. Every song is structured differently, but it will usually be some variation of this. Choruses are great because they act as a "hook" or something audiences can get multiple exposures to so that they can rap along with you for at least part of the song.
Revise. Once you have created a complete set of lyrics, critically analyze what you wrote. Just because it seemed brilliant when you wrote it, that does not mean that it will remain that way. Be merciless with yourself and always strive to write more interesting lyrics. It can help immensely to perform your hip hop lyrics in front of other people to get their immediate reaction.