Dan Zarrella loves to dissect vast numbers of social media posts to determine which language seems to be the most effective. His latest effort is summarized here:

New Data Shows the 7 Most Powerful Calls-to-Action for More Retweets

For me, the biggest surprises were that "please help" performed better than a direct call for a retweet and that "please retweet" did better than the more common "please RT."