Explain the bad grade to your professor. For example, if you misread the directions, let the professor know that this is the reason your grade suffered. He may consider allowing you to retake the exam.
Give a personal excuse. Many students face tough times while in college. If you have a legitimate personal problem, let your professor know. He may take pity on you and help you work on getting your grades back up.
Point out a grading mistake. Professors will usually change a bad grade if they made an error during the grading process. Point out any mathematical errors or answers you feel were marked off incorrectly.
Defend your exam answers. Essay exams give you more room to get creative with the reasons a professor should change a bad grade. Let them know you think your answer's right and provide evidence to back up the claim.