An email to your senator may result in a form letter response and a phone call to the office may amount to a tally mark on an administrative assistant's notepad.
But, for any given policy concern, if you want to get their attention a letter to the editor in one of your state's 5-10 biggest newspapers that mentions them specifically BY NAME is the way to go.
That is the crucial thing to know--the rest of this post is an explanation of why I know this is true.
I know this because, when I interned in the D.C. office of a senator one summer, one of the duties I shared was preparing a document that was distributed internally both online and in paper format. This document was made every day and comprised world news articles, national news, state news, and any letters to the editor in the 5-10 largest newspapers within the state that mentioned the senator by name. I was often the person who put that document on his desk, and it was the first thing he read every morning after arriving to the office.
I began to suspect that this was standard operating procedure because several other senators' offices share the same printer in the basement of the Russell Senate Office building, and I saw other interns doing the exact same procedures that I was involved in.
Since the internship, I've conferred with other Senate and House employees past and present and determined that most--if not all--offices use essentially the same procedure.