The Electronic Freedom Foundation has created the Trolling Effects project to help companies defend against patent trolls (or non-practicing entities if you want to be politically correct). The Trolling Effects website houses a database of troll demand letters so retailers can go to the site after receiving a demand letter and get some sense of the scope, practices and other information about the troll and its enforcement activity.
To be effective, retailers will need to actively send their demand letters to the database. However, this is not without risk. Sending your letter to Trolling Effects could expose you to the troll as a participant and some trolls may retaliate by filing suit more quickly than they otherwise would have or increase settlement demands. Industry experts suggest that retailers redact any identifying information before sending to reduce any chance of retaliation.
It will be interesting to watch how this project evolves. Chilling Effects, a similar project has been beneficial to the industry. I hope this project is similarly successful.