You must know or be seeking decisive assurance that you are called by Him to minister the Word; and you must eventually, before you begin, be so certain of this that you would die at the stake for your knowledge.
[Then] you must be willing to wait His will. Some of the most fruitful ministers I know in Scotland have had to wait years for their God-given appointments. And I might add that some have to wait for years in what I call a preparatory ministry, which is often more for their own personal good than for what an unwilling evangelistic people bargained for. You must be sure that you are in the right place. Only one thing kept me in my pulpit when all hell was let loose against me: it was the knowledge that God put me there, and there I had to stay until God took me out. I have hurled this more than once at my enemies with, I assure you, devastating effect!
(From William Still, The Work of the Pastor, pp. 117-118)