I recently found a method that works great. I'm sure there other, (possibly more elegant) solutions, but this gets the job done.
There are two functions to define for this solution; the best place for them is probably your .emacs file.
The first function, beginning-of-buffer(), from rattlesnake.com simply moves the point to the beginning of the buffer, leaving the mark wherever it happens to currently be.
"Move point to the beginning of the buffer."
The second function, clear-buffer(), from Audacity Init-Nyquist, deletes everything in the buffer and moves the point to the beginning of the buffer.
"Kill all of the text in the current buffer."
(clipboard-kill-region 1 (point-max))
It is of course possible to combine these two functions together if there is no reason to re-use beginning-of-buffer(). Once this is in your .emacs file, or has been run in Emacs, you can type M-x clear-buffer to clear the current buffer. It can also be mapped to a key combination like any other function.