Obtain copies of practice writing sample questions. The Law School Admissions Council (LSAC) which administers the test offers links to free sample tests online, or you may purchase LSAT practice tests from a number of sources.
Read over the instructions and the sample questions carefully before attempting to answer them. Familiarize yourself with the guidelines set forth by the LSAC. When reading the questions, try to pick out keywords or concepts that pertain to the argument you plan to make. Create a rough outline or list of phrases and ideas as you go.
Take a practice test under simulated testing conditions. On test day, you will have 35 minutes to complete the writing sample. In addition to the test itself, you will be given an answer page and a sheet of scratch paper. Find a quiet spot and stage a dry run by writing a response to one of the practice tests within the 35 minute time frame.
Have someone read your sample essay. There are services that will provide a detailed critique for a fee, but you may consider asking one of your current professors or a classmate to read over it for you. Choose someone you trust to provide you with an honest critique. Ask them to point out any spelling, punctuation, or grammatical errors as well as errors in logic.
Continue taking timed practice tests until you become comfortable with the format. Doing so allows you to get a feel for the pace at which you should be writing and it will help in reducing anxiety on test day.