In one sense, this is true: your writing skills are unlikely to substantially change through short-term preparations. However, there are some things you can do right before test day to help you to get a higher score.
One of the most effective is reading up on topics in science, art, and history that might apply to the writing prompt you will see. (Click here for recent prompts from the SAT and here for a sample prompt from the ACT.) How exactly does this help? Both tests give higher scores to students who write longer essays—i.e., essays with a greater number of supporting examples. On test day, you'll want to spend your time writing rather than thinking up examples.
Click here to read more from CollegeAdmissionsPartners.com.