There are lists being pulled together to help focus the boycott efforts. One is here on Google Docs (but is lagging behind on being updated. The last official list of SOPA supporters was posted in December 2011 and can be found here).* Near the top of the Google Doc list is a link to a boilerplate script you can use to contact the individual companies you will be boycotting, to let them know your opinion and that you will not support their company unless they rethink their position. And there even is an app to help you identify the companies and their products.
So whether you favor a total boycott or a targeted boycott, you can participate and you can tailor your level of participation. We have the tools, the will and the strength to take a stand.
*Note: As an example of the challenges of coming up with a final and accurate list of supporters, note that the Business Software Alliance that represents a large number of software and hardware companies (Dell, Apple, Intuit, Adobe) withdrew its support in November 2011 but still is listed on the Google Doc list. So when in doubt, you can always check with the company in question to determine their latest stance on SOPA if you do decide to pursue a targeted SOPA boycott.
edited to add: name of the targeted campaign, along with a link to the Android app.
edited to highlight text in the original post that pointed out that the lists of SOPA supporters are in flux and not up to date. Once the SOPA and PIPA bills were withdrawn on Friday January 20, 2012 the House Judiciary Committee removed the lists of supporters.