Here are some tips: 

• If the internship posting does not tell you specifically what to use as your email subject line, use this format: “Application for [specific quarter and year] [internship position] at [XYZ company]”.
• Address the internship coordinator by name if possible.
• Briefly introduce yourself and restate what internship you are applying for.
• List all of the applicable application materials you have attached to your email.
• State what time you will be in Washington, DC and available for the internship.
• Thank them for their time and consideration, and say that you look forward to hearing from them.