Once you've done your job interview, you should follow up with a formal letter thanking them for interviewing you, and to ensure they know that you are eager to accept the position should it be offered to you.
You would be surprised how many people don't do this - and it really helps you to stand out from the other applicants.
A thank you letter for a Medical Assistant isn't really much different to one you would write for any other kind of job - here is a sample that will give you a good starting point for when you write your own: