Purchase a preparatory book specific to the test you are planning to take. These study guides are easy to find in a local bookstore or on an online retailer. These books usually have practice tests inside. Take the test, focusing on the writing portion. Some books will have entire chapters dedicated to writing prompts with which to practice.
Practice online. For example, the College Board, the administrating company behind the SAT and ACT tests, has practice questions and writing prompts on their website, collegeboard.org. The official websites for each standardized test generally has sample practice questions.
Have a peer review your work. Try to find someone who excels in English and writing. Ask him or her to give an honest critique of your essay. Another idea is to ask a teacher to look over your work.