In 2008, Erica Chenoweth published a study comparing 323 violent and non-violent civil resistance campaigns between 1900 and 2006. Among her findings:

• Non-violent campaigns worldwide were twice as likely to succeed as violent insurgencies.
• This trend is increasing — in the last 50 years civil resistance has become increasingly frequent and effective, while violent insurgencies have become increasingly rare and unsuccessful.
• Campaigns were successful once they'd achieved the active and sustained participation of just 3.5% of the population — and many succeeded with far less than that.
• Every single campaign that surpassed the 3.5% threshold was a nonviolent one.
• Campaigns that relied solely on non-violent methods were on average four times larger than the average violent campaign — and much more diverse and inclusive.

Erica presented her research at TEDxBoulder last month:

She's also posted an FAQ and a transcript of her talk along with links and footnotes.