In an article in a recent issue of the NY Times http://www.nytimes.com/2012/05/20/opinion/sunday/who-arrived-in-the-americas-first.html?nl=todaysheadlines&emc=edit_th_20120520, they asked the question:
WHEN and how did the first people arrive in the Americas?
And the article says that "We now know people were in the Americas earlier than 14,000 years ago. But how much earlier, and how did they get to a continent sealed off by thick sheets of ice?"
I don't know for sure, but I'm going with really cool, fast sleds.