Determine what makes you unique. Admissions staff must read thousands of personal statements every year. If you can write an essay that stands out from the pack (in a good way), you will be more likely to get accepted. What makes you unique may be a disability, the struggles you face as a minority, your advanced experience on archaeological digs or any other personality trait or life experience.
Pick a topic. Some schools suggest the topic, while others allow you to choose your own. Perhaps you would like to write about the experience that made you want to study history or archaeology or your career aspirations. Whatever you choose to write about, it should be something that you are passionate about. If you are excited to put the words of your essay to paper, that is a good indicator that you have found a good topic for your essay. You may also want to contact a professor or administrator in the department to see if he has any preferences or requirements about the topic of your essay.
Review the school's requirements for length and spacing for the essay. Do not exceed the word limits set forth, and make sure to use proper spacing and formatting. Review both the admissions website, as well as the archaeology department's webpage, for relevant information.
Write your first draft. It doesn't have to be perfect. It might be too long or too short or lacking a personal touch. You can fix these problems later. For now, just focus on getting the draft complete. Your essay should show off your knowledge of archaeology (dig site protocol, famous archaeological finds, etc.), but it should be not cluttered with bits of archaeological knowledge just to make you seem smart. Include only what is relevant to your topic.
If you have time, put away your draft for a week. When you revisit your essay in seven day's time, you will have a fresh, objective perspective on the piece. If your essay pertains to a specific archaeological discovery or famous archaeologist that inspired you, it might be prudent to spend that week reading related books or articles about your topic.
Review your first draft. Each essay is different, but all effective essays should be vivid, concise and memorable. Rework your first draft for clarity, length and reader appeal. As you review your essay, try and think like a person who is reading these words for the first time. Make sure that your essay shows your passion for the field of archaeology, as well as some knowledge of the basic history or practices of the discipline. Don't worry about sounding like an expert; just show you have both some information and a willingness to learn more about the field.
Get a second opinion. Have a teacher, counselor, parent or friend review your essay, and ask for her opinion. If you have cultivated a relationship with a local college archaeology professor, historian or other archaeological professional, consider their feedback as well.
Consider your feedback, and prepare your final draft. Review it one final time, looking for spelling and grammatical errors, as well as for any improvements that you can make to the piece to make it more authentic or engaging. Make sure that you have correctly spelled any terms, place names or names of archaeologists used in your essay (for example, Tenochtitlan, Heinrich Schliemann and so on).