Motivation and goal setting
You might be asking what motivation and goal setting have to do with one another. In fact, the two have a lot in common. Setting some goals will actually provide the driving force behind the motivation. With unclear goals you have no where to begin, and no where to end. So let's get started.

• Set some very specific goals. Since we're talking about getting your grades back up to par, you need to be very specific in the goals you set for yourself. Look at each one of your classes and figure out what exactly you need to do in each one to get the grade you need. Talk with professors and ask if you can do any extra credit work, or spend extra time each week studying each subject. Whatever it is, just be specific. Otherwise you have no way of working your way up to the end result; better grades.
• Document your progress. Documentation is an essential part to finding the motivation to do better. It's one thing to have a goal and work towards it. Achieving it is another thing. Most people work much better if they have a way of seeing their accomplishments along the way; something to look at. There are several ways you can document your progress. You can record your progress in a journal of some kind. Write down your beginning grades and then your goal of what you want them to be. Then write down each specific step you are taking in order to reach your goal. Each day, week, or month look through the journal and check off those things you have accomplished. Seeing how successful you have been can help you stay motivated to keep working hard. Documentation can also help with areas that you may need some improvement in. There may be a class that you haven't given the time and effort needed. Documenting it will help you see what needs to be changed in order to reach the goal.
• Give yourself a realistic time-line. When you're trying to improve grades you generally have a specific time-line of when the grade needs to be improved. Whether it is a semester, two semesters, or the entire year you need to assess your goal and decide whether or not you can realistically accomplish it. If you have an unrealistic goal it's going to be impossible to reach, thus killing the motivation to achieve it. In order to stay motivated, make your goals and your time-line to complete them realistic.
• Be flexible. Flexibility is another major part to staying motivated. Getting good grades doesn't come easily, however with some hard work and goal achievement it can be done. Sometimes during the process though, there are set backs and disappointments. You need to be somewhat flexible with yourself; otherwise you will never be motivated to improve.

Figuring out how to get motivated to improve your grades is the easy part. The hard part is actually taking the initiative to do it. Good luck.