Dear ____________:
Thank you for you interest in _______________________. I am always interested in hearing from my congressional representatives on issues of concern to them. While we may not always see eye to eye on every political point, I can assure you your input has been noted and that it is important to me.
Now go away.
Sincerely ...
Then include a promotional flier or a signed picture. Or a Capitol keychain.
That's great! I should file that letter somewhere for all the constituent mail that comes my way.
I would only add "Now shut up and go away."