While more difficult than sending them a letter, visiting your Members of Congress, or their staff, face-to-face is the most effective way to actually influence them.
According to the 2011 Congressional Management Foundation’s report Perceptions of Citizen Advocacy on Capitol Hill, personal visits by constituents to the Washington or district or state offices of members of Congress have “some” or “a lot” of influence on undecided legislators, more than any other strategy for communicating with them. A 2013 CMF survey found that 95% of the Representatives surveyed rated “staying in touch with constituents” as the most critical aspect of being effective legislators.