After interviews - if permitted by the institution - there are several ways you can communicate with a medical school or residency. My favorites are brief thank you notes and well-written letters of interest.
But what about the "second look?" I've had many mentees ask me if re-visiting a school or training program after the interview day is valuable. Unfortunately, there may not be a definitive answer to this question because how a second look is perceived varies by institution.
The cons of the second look are cost and time. And perhaps even worse, there is the risk that you are imposing on the school or program. You don't want your request for a second look to work against your candidacy. On the other hand, the pros of a second look are a demonstration of interest and enthusiasm that many schools and programs are seeking.
If you are a pre-med and are wait listed at a medical school, I would generally recommend a second look if you can swing it. After all, you have every right to visit the institution you may be attending. If you go for a second look in this scenario, make sure to do a formal visit: Let the admissions office or Dean's office know you would like to spend the day and ask if you might have an opportunity to meet with students and even an admissions officer to support your candidacy.
If you are a residency applicant, it's hard to say if a second look will help or not. A residency director friend of mine says that a visit from someone who travels from far away might improve a candidate's standing by a few slots on the rank list at her program. That sounds like a tepid endorsement at best... If you are considering a second-visit, make sure to speak to the program coordinator. S/he might advise you against it or, on the contrary, let you know it's appreciated by the residency director.
As we all know, the residency and medical school interview scene is already stressful and expensive, so unless you are a wait listed pre-med - with all other things being equal - I generally would not push yourself hard to do that second look.