Not all teachers are created equal. Some tend to give higher grades than others or be more lenient with deadlines and scoring. Find out who these teachers are and take classes from them if you can. Many universities have anonymous professor review sites. If your high school or college does not, ask other students to find out about teacher policies regarding grading, absences and late work.
If you enjoy a class, you are more likely to do the work and get a better grade. You will be more motivated to speak up during lecture and hand in your papers on time. If the professor thinks you like the subject, he will be more inclined to like you and maybe grade you slightly higher. In college, you have a choice for most major requirements and school-wide requirements. Take classes that interest you and motivate you to learn.
If you have to fulfill a general requirement, find out which classes have the least amount of work or are the easiest and choose these. If you don't know anything about the class, use your common sense. A general music class is probably going to be easier than an introductory art history class if you don't like to read and write. If you like to work with your hands, then a crafts class is better than either of these courses. If two classes fulfill the same requirement, take the easier one.
Many colleges and universities look at your weighted GPA, that is your GPA when your classes are weighted or multiplied by a coefficient. This rewards students for taking advanced courses, which might be harder than regular classes. If you think you can handle a few of these courses, take them. Doing well will be rewarding and raise your GPA more than a normal class would.