using convert to batch convert

[Aaron Brashears <aaron@gila.org>]

I have many, many images that I want to scale down using imagemagick's
convert utility. The convert utility doesn't seem to work correctly
unless you give it an output name per input filename, but the man page
doesn't make it clear how to do this, and I'm stumped as to how to
generate a new name for each input file name.

The command I would expect to work:

convert -sample 50% *

only converts the last file in the directory, and converts it twice
(!) to a filename with '.0' and '.0.0' appended. Any way I can use
convert in some kind of batch mode?