Electronic Frontier Foundation has offered a guide for using and protecting your cell phone and data while you are protesting. Here's the introduction to this helpful article by Eva Galperin:

Protesters of all political persuasions are increasingly documenting their protests -- and encounters with the police -- using electronic devices like cameras and cell phones. The following tips apply to protesters in the United States who are concerned about protecting their electronic devices when questioned, detained, or arrested by police. These are general guidelines; individuals with specific concerns should talk to an attorney.

From the Internet to the iPod, technologies are transforming our society and empowering us as speakers, citizens, creators, and consumers. When our freedoms in the networked world come under attack, the Electronic Frontier Foundation (EFF) is the first line of defense. EFF broke new ground when it was founded in 1990 — well before the Internet was on most people's radar — and continues to confront cutting-edge issues defending free speech, privacy, innovation, and consumer rights today. From the beginning, EFF has championed the public interest in every critical battle affecting digital rights. Blending the expertise of lawyers, policy analysts, activists, and technologists, EFF achieves significant victories on behalf of consumers and the general public. EFF fights for freedom primarily in the courts, bringing and defending lawsuits even when that means taking on the US government or large corporations. By mobilizing more than 61,000 concerned citizens through our Action Center, EFF beats back bad legislation. In addition to advising policymakers, EFF educates the press and public. EFF is a donor-funded nonprofit and depends on your support to continue successfully defending your digital rights. Litigation is particularly expensive; because two-thirds of our budget comes from individual donors, every contribution is critical to helping EFF fight — and win — more cases.