Scoring GRE analytical writing practice essays
You've got a few options:


See if your school has a GRE preparation course (you may have to pay for this) or other writing tutorials.
As Tirath said, look at ets.org for strategies.
If you're still in undergraduate school and have at least another semester, enroll in a writing class. The practice you get from the class should improve your overall ability.
Look online for test preparation courses. These can be expensive, but sometimes they have "guaranteed results," and you can probably figure out the best ones by looking at reviews.
Advertise on Craigslist (or similar) for a tutor. If you are in a college town, you may find someone willing to tutor you relatively cheaply.
Look online for GRE writing samples, and read as many as you can.
Practice writing every day. The more you practice, the better you'll get (and read as much as you can, too).


Keep in mind that while you always should strive for high scores, your field of study will partially dictate how important the writing score is. If you're going for a history PhD, you'll need a high score, but for a science degree this tends to be less important (for admissions--being able to write is important for any PhD!).