Until recently I used Eclipse, and in Eclipse I simply created a folder /res/layout-landscape and put alternate landscape layouts inside.
However, it seem in Android
MrEngineer13's answer is spot on.
The only thing that I would add is that his screenshot may lead you to believe that you should be selecting 'UI Mode' from the 'Available qualifiers' list.
Just to clarify: after filling out the Directory name and Resource type, select 'Orientation' from Available qualifiers, which will lead you to the screenshot that MrEngineer13 provided.