documentation for unmarried partners

What type of supporting evidence is required for unmarried partner visas?  Do we have to have been co-habitating for 2 years?  Or just prove we've been in a committed relationship for 2 years.

You will have to prove you have been living together in a relationship akin to marriage. So you will need documentation of post to each of you to the same address over a 2 year or more period.

Be sure to study up on the new joint commitment grounds.  I pointed it out when it first surfaced here...

http://talk.uk-yankee.com/index.php?topic=49318.msg941319#msg941319

Despite my prediction that these grounds couldn't persist, we're still getting refusals in NYC based upon these grounds.

Forum gurus are respectfully asked to add this to advice.