If you want to have any hope of inspiring somebody, then you must be prepared to give as much of yourself as you possibly can to them without ever expecting anything in return, even if they don't respond to your preaching. By preaching without attachment to the results, we can remain inspired to preach in any condition, whether somebody is receptive or not.
Taking Care of Devotees, p.179, 1st paragraph (from a letter to a disciple)