Why do my files keep changing once I add them to my MP3 Player?

I have a lot of files in my Windows Media Player Library and I set them all up to how I like with the correct Genre, title, artist etc. Once I add them to my MP3 Player some of the files change what I had customized them to be. Even if I look up the album info and change the artist, album, name that way they still have different information then what I wanted and very rarely do they have the correct album art. How can I get my filses to stay the way I have set them when adding them to my MP3 Player?