How can a preacher train his congregation to make the most of expositional preaching?

Preach at a level that’s high enough to make them stretch, but not so high that the sermon flies over their heads.
Apply the text to their lives specifically.
Provide cards that list all the upcoming sermons, a blog post, or something similar that will inform the congregation of what biblical texts the preacher will cover in coming weeks. Encourage the church to study the biblical texts during the week before the sermon.
Encourage them to discuss the sermon at lunch on Sunday or in their small groups.
In your sermons, draw attention to Christ, not your personality. Work to win people to Christ, not your personality or humor or cleverness or intellect.
Remind people to pray specifically for the general preaching of God’s Word in your church’s prayer meetings.
From time to time, teach and preach about God’s purposes for preaching.
Make your number one goal as a pastor to prepare excellent sermons. If you’re not the full-time preaching pastor, do what you can to enhance and affirm the effects of the preached Word through your particular capacity in the church.