for f in * ; do mv "$f" "PRE_$f" ; done

In my case I wanted to rename all sub-100 filenames to have an extra zero so sorting played nicely with filenames beginning with 100+. To accomplish this I found about the rename command (thanks to this site.)  The command I used to enforce natural sorting was the following:

rename 's/\d+/sprintf("%03d", $&)/e' *

The command looked for anything beginning with a number, then used sprintf to make the number 3 digits. The asterisk instructed the rename command to work on every file. Success.