Get to know your student by asking for a student resume or information sheet. Most often, your school's counseling department provides a form for students to fill out detailing their accomplishments (awards or elected leadership experiences), experiences (service projects or mission trips), work history, memberships and attributes that set him apart from his peers (family situations or unusual skills). These pieces of information will gives you something to highlight as you write your letter. (For a sample form, follow the link in the Resources section.) Your interactions with the student may be limited to one context, so having this resume or information sheet will help you write a more well-rounded letter.
Follow the requirements set out by the university. There may be a form or a questionnaire that must accompany the letter. In submitting letters of recommendation, some institutions want them mailed separately, others include the specific request that they be mailed in a sealed envelope with your signature across the seal and other institutions request they be enclosed with the rest of the student's application materials. If your student did not specify the requirements, contact her and request this information.
Introduce yourself and explain your relationship to the student when beginning the letter. This will communicate to the reader the context from which you are recommending the student. Detail the number of years you have known the student and in which ways you have interacted with him.
Summarize the student's personality. Don't waste your time simply regurgitating the student's resume, as the application board will have this information through other sources. This is your chance to move the applicant from facts and figures on paper to a live person with various personality traits. Give examples of specific encounters you have had when possible. If you write that the student is generous, include a story where you observed her acting in this vein.
Describe a skill the student has or an activity he has participated in. Breathe life into an item on the student's resume by describing the item in detail and giving any related anecdotes you may have. Some letter writers go an extra step and speak with a student's coach or other adult to obtain anecdotal evidence that way.
Suggest that your student will make a valuable contribution to the school's community, will devote himself to learning and will enter the "real world" as an exemplary model of the university. Invite the members of the application board to contact you should they have any further questions about the applicant and enclose your contact information.