How to get my puppy comfortable with her harness?

I have a 3 month old chihauha, and i would like to take her out for short walks but she gets a real attitude when the harness is on and is like a lump on a log. She fights us when we put the harness on. I want her to know that the harness is safe and not something to be scared of. How can i get her to wear the harness and forget all about it. She can't wear a collar becuase it can hurt her trachea. Please help.