When I was young I remember my parents using some sort of glue product that went from the tip of one ear to the other that held the ears up. After a while they got rid of the stuff and the dog's ears stayed up after that. Sorry it's a vague description but it's 3 am and it's the best I've got at the moment :P
Yorkies ear are not meant to be cropped they will either stand on there own after they are down teething because during this process they're ears do tend to flop downward but it is common to have a larger maybe 6pounds up for the ears to flop . I have a floppy ear yorkie and a yorkie who's ears stood up on they're own after they finished teething around 5 months be patient and love them either way a lot of times people who tape the ears end up damaging cartilage and causing the ear to never be able to stand on its own. Love them no matter what! ;)
I would ask AnnFalkDVM on this site. She is very knowledgeable in this field as she is studying to become a veterinarian or already is one. Either way she is a wonderful resource on this site. Myself, I wouldn't use glue or glue products on an animal.
It all depends on what breed of dog it is, if it's ears will stand up, some ears aren't meant to stand up, and some weren't meant to flop over! It depends on the breed. :]
It depends on your dogs breed but if it senses danger or hears something past than humans can hear it MIGHT as I said it depends on the breed.