Probably add something like this:
if [ "`id -u`" -eq 0 ]; then
export PS1='\u@\h:\w\# '
else
export PS1='\u@\h:\w\$ '
fi
That's adapted from the /etc/profile on my Debian box, so it ought to be good.
EDIT: That assumes your shell is bash. If not, it likely won't work, and you'll have to adapt the one above using this format.