there are 2 measures of quality, visible and technical... generally youtube clips are heavily compressed as mp4's which is an mpeg format. by it's nature, you are usually decompressing it to work with it and the recompressing it to export it and there will be measurable loss but not necessarily visible loss. It's very easy to over obsess over this kind of thing. Generally the safest way to work in premiere is to select the clip in the bin and control click and choose new sequence from clip. This will match pixel dimensions and frame rate.. When you go to export, choose the youtube preset that matches those settings and you should be good to go. visually compare the original file and your new exported file and I doubt you'll see any difference. If you do, post back with your source properties and maybe someone will have some ideas about how to improve the quality. there are other editing apps which supposedly do not decompress and recompress... not sure how true that is, but as I said, this is something you probably don't need to worry about. And remember that everyone's viewing experience in youtube is different. If the end user doesn't have the bandwidth to play the clip at full resolution, youtube will playback a lower resolution...