On a larger scale, organizing a sort of free internet seal of approval for all representatives could be even more powerful. If representatives know that voters would be more likely to vote for candidates with the seal of approval, then they might be persuaded to earn that seal.
Edit: I just wanted to add that while generally I agree that writing letters has little to no effect, I think it's important for people to start DOING something. If it starts with writing letters, maybe the upshot will be that people are embolded to participate in bigger future actions, like taking part in demonstrations and more when the time comes. Cynicism and defeatism aren't the right starting attitudes for a fight. Just my two cents.