I think writing them is the best impact - better than an email or phone call.
I suggest pre-stamped post cards (tip-o-the-hat to Calypo Jones for that little gem). Buy some and keep them in your desk or wallet. When writing Senators, use this format................
The Honorable (full name)
United States Senate
(Room #) (Name) Senate Office Building
Washington, DC 20510
Example:
The Honorable Rand Paul
United States Senate
124 Russell Senate Office Bldg.
Washington DC 20510-1703
...............the staff will know you're respectful and informed and that may help you get attention from the many other pieces of mail. Also, CJ pointed out to me that if you use a post card, your mail goes straight through. Letters have to be sorted, opened, and checked for Ricin or whatever - so they go to a big sorting facility before being sent to the Senator you've written.
UPDATE 03/12/17 Current list of US senators
https://www.senate.gov/senators/contact/