How do you get a baby to keep a pacifier?

My daughter is 3 months old and will not keep a pacifier in her mouth by herself. When she naps during the day she wants to have it in her mouth at all times. However she flips her head constantly and loses it then screams bloody murder. The second I put it back in her mouth she calms down. I can not get anything done during her naps because I basically have to sit here and hold it there. How do I get her over this? She has also been well fed before her naps.