Identify your problem area. If you are struggling with the mathematics portion, focus on improving your capabilities there. Improving your reading comprehension will improve your scores; many of the questions are word problems on all of the tests excluding the Language Arts: Writing portion.
Purchase and study books that were written to help prepare for the GED tests. There are many resources available online; as well as in libraries.
Take online practice tests. A multitude of websites offer practice tests dedicated to each of the five GED tests.
Contact your county's Board of Education and see if there are local classes available that you can enroll in to improve your GED scores. Many classes have flexible hours and are cheap or free to take.