The Spirit of the Disciplines: Understanding How God Changes Lives. This article is essentially Willard's summary of the book: http://www.dwillard.org/articles/artview.asp?artID=57
It's a false dichotomy, that Christians too often accept, that there are inward spiritual things we do and outward, physical things that aren't spiritual. It's an ancient problem in our thinking, going back to first centuries where people denied that Christ had an actual physical body like ours. We don't like to think of him doing "common" things like eating, working, scratching an itch. The human body is part of the Imago Dei. Exercising our faith requires physical deeds done in a physical body, so we can eliminate the idea that physical deeds can't be spiritual or worshipful, or even part of salvation-- "salvation is a life." We exercise proper dominion over creation with the power God has put into our bodies properly combined with the infinite power He provides.
Salvation is more than just mental ascent to forgiveness of sins through Christ. The NT leaves no room for that attitude and early Christians were more interested in the life of Christ and not just his death. (The cross didn't become a common symbol of the faith until after 400 A.D.) Redemption is about more than just our souls at the end of life, but about our bodies and our actions here and now. When we who are dead in our sins are connected to the Spirit, we become alive and our bodily actions are evidence of this.
Asking "What Would Jesus Do?" in a given situation sets a person up for failure because it focuses on a single act rather than the training behind the act. It's like forgetting a professional baseball player spends grueling hours training physically and looking over scouting reports to train for a pitch he finally sees-- because all we see is him swinging the bat. Jesus spent hours and weeks out of sight in prayer and fasting, and 30 years of his life we have little knowledge about, to train for those specific moments of which we do have a record. If you want to be like Jesus, then train like Jesus, don't just try to act like him "in the moment."
Willard references and recommends Foster's book on discipline (my review). This book is a much more theological underpinning of Foster's book. What are spiritual disciplines? Essentially they are activities that put us more in touch and fellowship with God.
There are some inward and outward disciplines, similar to Foster's list. Solitude for the purpose of being fully with people when you are with them is one. Study is, of course, important. He would not lead a group in spiritual exercises without requiring focus on memorization of Scripture.
Fasting.Since it is clearly an expectation of those who want to depend on God in both Old Testament and New Testament, this is something I need to start practicing.
Frugality is a discipline. Willard gives a lengthy explanation of this, delineating frugality from an intentional poverty. He pushes back against a modern interpretations of Scripture which exalt poverty, noting that poverty is not a guarantee of blessing or a way to receive grace, that is not a right interpretation of Matthew 6. "The worst way to help the poor is to be poor," he says. Willard gives a brief history of the development (and diversity) of thought on wealth among Christian teachers. He is quite critical of John Wesley's lament about how wealth ruined his Christian converts. He promotes the idea of redeemed business-- shouldn't we want Christians in positions of great influence on the management and distribution of wealth? We should want Christian businesses to succeed and to grow for that purpose, the more one has the more he has to give. This does not mean that Christians should inherently love their wealth, but rather that they should exercise frugality all the more-- anything that comes between themselves and their love of God is an idol that should be discarded.
Willard spends some time toward the end defending his positions, he seems himself pushing back against hundreds of years of errors mainly due to Western philosophy. He's not arguing for a completely aesthetic faith, but one that has aesthetic qualities. I don't really do the book justice as a whole in this review, so I recommend checking out the linked article about or just read the book.
This is a 5-star classic that I wish I'd read years ago, but probably wasn't ready to read it.