I consider mixing to be an artform that focuses on balance and perspective. A mix can be approached from countless angles--each as aesthetically valid as the next--and each determining the focus, quality, and character of a song.
When I approach a mix I first try to satisfy any specific requests from the artist. I like to spend time unattended, trying different things until I end up with something I think sounds good. Then I welcome feedback from the artist to see if I'm on the right track or if anything can be improved. I really enjoy the collaborative process and have no problems scrapping anything I've done and trying other ideas and approaches until it sounds right to the artist. I charge a flat rate per song for mixing so I can fully explore a mix without worrying about how much time it takes.
The mastering process can be as straightforward as encoding and delivering the correct files for different media release formats, or it can involve that plus lots of sonic shaping to create a more unified final product. Sometimes mixes need an extra push to get them to desired levels and impact, and I enjoy working on this type of creative mastering. Other times mixes just need a minor tweak or to be left alone, and if this is the case I'm happy to do that as well. It really depends on what the artist desires from the mastering process.
Regardless of the approach, I charge a flat rate per project including any revisions that may need to be made. I won't finish working on a project until the artist(s) are 100% satisfied.