Here is John Wesley writing to John Trembath (August 17, 1760), a young minister who was a poor preacher:

What has exceedingly hurt you in time past, nay, and I fear, to this day, is lack of reading.

I scarce ever knew a preacher who read so little.

And perhaps, by neglecting it, you have lost the taste for it.

Hence your talent in preaching does not increase.  It is just the same as it was seven years ago.  It is lively, but not deep; there is little variety; there is no compass of thought.

Reading only can supply this, with meditation and daily prayer.

You wrong yourself greatly by omitting this.

You can never be a deep preacher without it, any more than a thorough Christian.

Oh begin!  Fix some part of every day for private exercise.  You may acquire the taste which you have not; what is tedious at first will afterward be pleasant.

Whether you like it or not, read and pray daily.

It is for your life; there is no other way; else you will be a trifler all your days, and a pretty, superficial preacher.

Do justice to your own soul; give it time and means to grow.

Do not starve yourself any longer.

Take up your cross and be a Christian altogether.

Then will all the children of God rejoice (not grieve) over you, and in particular yours.

Quoted in D. A. Carson and John D. Woodbridge, Letters Along the Way (Wheaton, 1993), p. 169. The original letter can be read here.

HT: Ray Ortlund