Know your audience. This is the first rule of nearly every piece of writing and it is especially important in this case. Make the information as personalized to the class as possible. Note any important events that happened over the course of the year at the school, within the community or even internationally that could impact the graduates.
Draw on quotes to help motivate your audience. Because you are the one speaking, feel free to put some of yourself into the speech. Perhaps there is something a family member said to you that you have always kept with you, or maybe you have a favorite line in a book you read. These can help motivate others. You may consider reading other speeches for inspiration, but do not draw on them too closely -- you want your work to stand on its own.
Avoid clichés that will compromise the power of your speech. If you must borrow an old expression, present the idea in a new way that will best relate to the graduating class. Sometimes old, overused phrases can still be uniquely relevant if used correctly in the right circumstance. For instance, perhaps you are giving a speech at an acting college. Ending your speech by telling the graduating class to "break a leg" might be more meaningful than if used in a traditional university.
Keep them interested with humor. Sometimes opening with a joke can lure people in and break the ice. A little laughter from the audience can also help settle your nerves and make you feel more at ease. This is not absolutely necessary, but it can help grab the attention of your audience.
Offer hope and inspiration for the future. Graduates should leave the ceremony feeling inspired, accomplished, somewhat nostalgic, but still optimistic for their future. Your words could play a large role in instilling these emotions in your audience.