"As a former Senate staffer and redditor, I will tell you that the most effective course of action in this situation is to call the DC offices of only your own Senators directly. Speak to the staff assistant, who will take down your comments and log them into a tracking system. Make it clear to the staffer that you are calling in opposition to SOPA. In the briefing prior to the vote, the staffer assigned to SOPA will inform the Senator of the aggregate constituent sentiment as reflected by phone calls and letters. Their vote recommendation will be, in part, based on constituent sentiment. Most senators will make up their minds based on the information provided to them by their briefing staff.
The more phone calls this issue receives, the clearer constituent sentiment will be to these Senators. Take the time to call a few times a day if possible. This is by far the best way to make an impact. In person meetings will have the same result."
Quoted from a comment on reddit.