Practice. Go to www.military.com to take timed practice exams to figure out your strengths and weaknesses on the exam. Because you already have a bachelor's degree, you should be well-versed in how to prepare for exams. The ASVAB is similar to the SAT and ACT in that it is a timed exam, so it is important to take practice exams under actual test conditions to be sure you are able to finish within the allotted time.
Study subjects you are unfamiliar with. Having a bachelor's degree means you are probably familiar with basic science, math, vocabulary and reading comprehension topics. You may be unfamiliar with mechanics or electronics, especially if your degree is in a liberal arts or social science category. Go to your local library and pick up books on basic auto mechanics and electronics. If you are familiar with these subjects before you take the exam, you have a better chance to do well in these areas and raise your score significantly.
Rest. Get a good night's sleep before the exam, and eat a healthy breakfast the morning of the test. Many people spend so much time studying for the exam that they forget to take care of their bodies. Going into the exam well-rested and well-fed will help your mind stay sharp when you need it to the most.
Guess if you have to. The ASVAB does not penalize you for wrong answers, so it is in your best interest to guess if you do not know the answer. You should not leave any questions unanswered on the ASVAB. If you are running out of time near the end of the exam, fill in the rest of the answers to give yourself a chance to get some points and achieve a higher score.