I went ahead and emailed all of my senators. For people who have never written, emailed, or called their representatives before:
Generally they will just tally people who support or oppose a bill and then the congressional staff will present the information to a representative before they vote. Unless you're going to present some new and profound argument, it's generally better to just tell them right out where you stand in a couple sentences, so they can go ahead and tally you. I think pretty much everything there is to say about SOPA and PIPA has already been said and I don't have anything new to say, so I just went with a simple one sentence email.
Representatives are not required to consider the sentiment of their constituents when they vote but they really do consider it. This is probably one of the cooler features of the American government. For all the power giant advertising campaigns from corporations have, you would be surprised about what couple thousand letters can do. Actually, you really shouldn't.
Of course, I don't expect either of my congressional representatives to act on it, since they co-sponsered the bill. On behalf of New York I'd like to apologize for our representatives trying to break your Internet.