Here you want to make the claim that you鈥檙e a good candidate for this particular program.
Avoid 鈥淚鈥檝e always loved History鈥 introductions; that鈥檚 assumed at this level.
Focus on the periods, themes, and questions that drive your passion for History鈥攆or example, 鈥淚鈥檓 interested in exploring the interplay of class and gender in industrializing European societies鈥 or 鈥淚鈥檇 like to examine American foreign policy vis a vis Russia in the 1930s.鈥 Explain how your undergraduate training has prepared you to tackle this line of research.
Explain how your interests match both the strengths of the program to which you are applying and the research interests of specific faculty members.
A personal statement should not be a long document: 2-3 full pages tops.