Collect every bit of information relating to your dealings with the account and use them to prove who you are.
If you made payments into the account and you can send MtGox the verification documents to prove who you are then that would help.  Did you make any deposits or withdrawls? and what methods did you use? and what days?
How old is your account?
How often do you use it?
Where do you access it from?
What browser do you usually use to access it?
Did you use the same PC to access your account all the time?  If so then it is linked by the IP address of your PC.  This can change when you reset your router but it will change to within a range.  Go to the url http://mtgox.com/this_is_danish_and_my_username_is_xxx_help_pox.html  this will appear in their web logs and might match the IP address you last logged in with.  Do it.

The villains will be able to look through your email and trade history so you need to find things that are not listed in those places.
Bank accounts.  Letters to your home.  Photo ID, Locations of access.

Maybe you are the villain and you want to prove you own someone else's account or steal another persons.
I don't know.