Only preach one sermon at a time. Never preach two sermons at one sitting. Repeat after me ...
Why state this? I think a temptation in preaching to the lectionary is to preach a sermon on two or more of the readings. The challenge is to preach one sermon only. That is, to preach a sermon which majors on one reading and minors on the others; or a sermon which follows a single theme through the readings; or a sermon which combines the three readings together in one, single, pertinent message.
It is possible to preach two or more sermons within the one period of the service called, 'the sermon.' That, I suggest, is a big mistake. No matter how brilliant each of the sermons is, together they will undermine the persuasive effect of each.
Only preach one sermon at a time. Repeat after me.