Consider your audience. Do not simply address your letter to whom it may concern or to the admissions office as a whole; rather, look up the name of a specific person you met who works in admissions and address your letter to her. Sending your letter to a specific admissions officer guarantees it will be read by someone who can influence the decision made on your application. If you mail a letter to the admissions office as a whole, it will most likely be read by a receptionist or student employee and may never make it into the hands of an admissions officer.
Thank the recipient for his time. Ultimately, your letter should convey your gratitude to the admissions officer, letting him know how much you appreciated the chance to speak with him. Acknowledge the ways the open house was helpful to you, using the letter as an opportunity to express what draws you to the college and why you feel you would be a good fit as a student there.
Be specific. If you spoke to the admissions officer at length about campus housing options, mention how helpful this information was to you in your letter. If you particularly enjoyed her presentation on financial aid, tell her what points you found most interesting. Not only can these specific examples help an admissions officer to remember who you are, but this also shows your enthusiasm and interest in the college.
State that you intend to apply for admission if you intend to do so. Pointing out that you plan to submit an application for admission to the college will alert the admissions officer to keep an eye out for your application to arrive. While your letter cannot guarantee that he will review your application when the time comes, it can only be to your benefit to make a positive impression in your letter.
Use a professional, formal tone. Even if you had a casual discussion when meeting the admissions officer, do not assume you should take an equally casual tone when composing your letter. The letter you send is an opportunity to present yourself as a viable candidate for admission to the college. Use the letter to show that you are a mature, educated individual who is ready to receive a college education.
Proofread your letter. Take the time to ensure there are no spelling or grammatical errors in your letter. Ask a teacher or parent to look over the letter and provide feedback before sending it. Even minor errors can make a bad impression with an admissions officer who communicates with hundreds of potential applicants each week.