In the past, beginning April 1 Case will contact you weekly to see if you are still interested. So wait until you get an email about getting on the waitlist and respond in the way that they tell you to maintain a spot on the waitlist.
As to a Letter of Continuing Interest (LOCI) I would suggest writing to your admissions counselor (Contact | Undergraduate Admission | Case Western Reserve University) and letting them know that you would attend Case if admitted (if that is the case) and why you want to attend Case.
For example:
"Beginning April 1 and throughout the course of the spring, we will be in touch with you weekly via email to provide you with an opportunity to keep Case Western Reserve informed of your plans and your interest in remaining on the wait list. If we are able to offer admission to any students on the wait list, we will use the responses to these emails to identify interested students, so it will be important that you respond each week if you remain interested. When we contact you, I hope you will elect to remain on the wait list.
We remain optimistic we will be able to offer admission to a number of qualified students from the wait list. In recent years, Case Western Reserve has admitted as few as 40 and as many 800 students from the wait list. We have begun offering admission to students on the wait list as early as April 15, but offers are often extended in the final week of April and throughout the month of May.
As a reminder, all students who are admitted from the wait list:
are considered for merit scholarships (about 75% of the U.S. Citizens and Permanent Residents who were admitted from the wait list last year received a scholarship)
are considered for need-based financial aid if they would otherwise qualify
are guaranteed on-campus housing"