If you want the new YouTube layout before it's rolled out officially you just have to follow this 4 simple steps:
1. Go to http://www.youtube.com
2. Press Ctrl + Shift and J in Chrome to open the Developer Tools
Press Ctrl+Shift+K in Firefox
3. Click on the "Console" Tab and enter
document.cookie="VISITOR_INFO1_LIVE=ST1Ti53r4fU";
4. Reload the website and enjoy the new layout.