I use --cut-dirs=# where is the number of directories before the one you want.
For example, if I want to get the SRPMS from this site, that has 9 directories before it.
Code:
wget -cm -nH --passive --cut-dirs=9 \
ftp://mirror.dulug.duke.edu \
/ftp.redhat.com/pub/redhat/linux/enterprise/3/en/os/i386/SRPMS