What I do is redefine "make_resolv_conf()" in the file
"/etc/dhclient-enter-hooks":

#!/bin/sh
#
make_resolv_conf () {
        echo search $new_domain_name >/etc/resolv.conf
        for nameserver in $new_domain_name_servers; do
                echo nameserver $nameserver >>/etc/resolv.conf
        done

        # Allow rewriting by pppdialer suite.
        #
        chown root:dialer /etc/resolv.conf
        chmod 664 /etc/resolv.conf
}

This file must be executable.  There is also a corresponding file
/etc/dhclient-exit-hooks which allows for further customization.  Just
read through the shell logic in /etc/dhclient-script to figure out how
to use them.

     -- Johnny C. Lam <lamj@stat.cmu.edu>
        Department of Statistics, Carnegie Mellon University
        http://www.stat.cmu.edu/~lamj/