To win one of the nice-looking controllers, you'll need to attend PAX East in person this weekend, take a photo of one of the controllers (they are on display at Microsoft's booth), and tweet to @Xbox and include #XboxOne and #XboxSweeps in your message.
Microsoft has also produced a custom Xbox One based on The Witcher 3. This system is also on display at Microsoft's booth (we'll upload a picture here when we get to the show floor), and the rules for winning one are similar. Take a photo with it and send a tweet to @Xbox making sure to mention #XboxWitcher3Sweeps and #XboxOne in your post.
Alternatively, you can enter by mail. Microsoft will give away ten custom controllers in all, but only one Witcher 3 console. The full contest rules are available here.
PAX East runs March 6-8 in Boston, Mass. GameSpot will be reporting from the show all weekend long.