There are some steps that have to be followed to do this:
-Get a hotmail, yahoo or any e-mail address that is not registered with any gamertag(make sure to put that you are from the year 1985 or older, otherways they will ask you for another e-mail to register the account later on).
-Go to xbox.com and register that account. Just say that you are from the country the demo(s) was/were released(I picked Japan because that's were most demos that are not released in North America are released).You will have to get any adress from the country you chose.
-After completing the registration just go to your xbox 360 and recover the gamertag. Complete every step to recover the gamertag and there you have it. That's all.
There are some fun demos like Viva Piñata, Rainbow Six: Vegas, Chromehounds, and a few others.
Ps: For those that don't know, all of the demos are showed in every account, so you can play the new demos in the others accounts.