Like most standardized tests, taking LSAT practice tests is a good practice for LSAT takers. Practice tests simulate the actual testing environment and give you insights into the various question types and sections in the exam. Most serious test takers take many practice tests before the actual exams.
Take LSAT practice tests after you have covered enough preparation ground. Taking an LSAT practice test too early may demotivate you. You can take one half way through the preparation stage. Take them more extensively in the final lap of the preparation phase.
You should take them very seriously and in the same rigorous conditions in which you will take the LSAT. Make sure that you take the exam in strictly timed conditions. Give yourself strictly 35 minutes for every section.
While you stick to reputed preparation companies for LSAT practice tests, it is always a good idea to use the material provided by LSAC, the makers of the test. They offer questions administered on the previous LSATs and are pretty close to the real deal. Make sure that you use some recent tests as well.
Also, take into account the extra experimental section during the actual LSAT. In the LSAT there are a total of 5 sections where four are scored while one is unscored. Make sure that you add an extra section in the LSAT practice tests to mimic the exam scenario. It will help you build stamina and confidence for the exam. Taking just four sections every time in the practice tests puts you in that mental zone and you are just not ready for five sections on the day of the exam.
Finally, use the practice test results to understand your own abilities and work on your weak areas. Analyze your section scores and tweak your study plan accordingly.