This may just be me being stupid but what did The Doctor tell her

Basically: 'something borrowed, something blue'. He implanted it so she would remember it on her wedding day, and he told her to love Rory.

Something old, something new. something borrowed, something blue.

told the story of the tardis ....... something old (the tardis is ancient), something new (the tardis is also brand new), something borrowed (the Doctor stole or rather borrowed the tardis) and something blue (well thats its colour)
all this built up into Amy's wedding day to help her remember the doctor and bring him back

I wonder how long ago Moffat came up with that idea, and how difficult it's been keeping it to himself all this time

Yes, it was genius really. And when he saw Amy in the forest and told her to remember he hadn't actually been back in time and told her what she had to remember, which is why he said it didn't matter. She just had to remember. So he then went back, saw sleeping Amy and told her the story of the Dr, knowing/hoping she would one day marry Rory and that the story of the Tardis being old and new and borrowed and blue would trigger her memory on her wedding day. And it did.

It was funny, trying to remember what he told her, thinking that maybe it had something to do with her sitting on the suitcase and hearing the tardis and now we know that at that point, in the forest he hadn't actually told her anything, crafty.

Live well, and love Rory

And - something old, something new, something borrowed, something blue.