Thank you, everyone, for your feedback. You've given me a lot to think about the past few days.
I've taken what you said and crafted it into my new copyright clause. I know you're not lawyers, but I'd be glad to hear what you think:
6. Digital Files, Prints and Copyrights
The photographs, digital files, or prints produced by the Photographer are protected by International Copyright Law (all rights reserved) and the Photographer retains the copyright to all photos and images produced. The Client may receive a copy of the digital files, watermarked, in a reduced resolution if it has been agreed upon as part of the Assignment Package. The Client also may choose a number of digital files (determined by the photographer) <what is determined by the photographer? Which files, or how many?>, to receive the full resolution of, free of charge, as a courtesy. Additional digital files may be chosen at a rate of $75/file. Alternatively, the Client may purchase prints directly from the photographer at a reduced rate <reduced from what?>. The client is granted the non-exclusive right to reproduce or print these files for personal/non-commercial use but not to alter them in any way. However, the Client agrees to obtain written permission from the Photographer (subject to additional charges) prior to the Client (or its assigned agents) publishing the photos commercially, or selling the photographs for profit.