Ernesto Falcon is the Legislative Counsel for the Electronic Frontier Foundation. Before that, he worked in various legislators' offices. As such, he knows a thing or two about communicating with members of Congress. He has a lengthy and detailed post up at EFF.org entitled "How to Talk to Congress." It's worth reading in full.
Specifically, I would like to call out this part:
Every letter, phone call, or email you send is absolutely critical because frankly, most people do not take the time to contact Congress. When people do rally in sizable numbers, no amount of special interest and campaign contributions can override the perceived opinion of voters back home and how that impacts an elected official's electoral concerns. The more confident a member of Congress feels in the number of people who will vote for them back home if they vote their way, the more resistant they become to opposing influence.
I think it is really easy to decry the corrosive influence of money in politics, declare that all politicians are bought and paid-for by Big Ag or Big Pharma or Big Oil or the Koch Brothers, and throw one's hands up in despair. I find myself falling into this trap. Calling your members of Congress (or writing them letters or emails) doesn't take all that much time, but it does require energy and attention, both of which can be scarce resources. And it's not just the time and effort of talking to your MoCs—it is also the mental bandwidth to keep track of and prioritize what you want to talk to them about.
But this stuff really does make a difference, and it's really important. If you are not calling and writing to and talking with your members of Congress, there is no counterweight to the campaign donations and lobbyists.