One of the tensions which rears its ugly head a lot lately (at least
in my experience) is the inability to reconcile spiritual disciplines
(like a regular personal prayer or Bible study time) with the freedom
we have in Christ. In this sermon, John Piper takes the “legalism” fang out
of it, by putting things in a right frame. It is Biblical wisdom
fleshed out in a very fine way on a most important topic. Do take the
time to listen. You’ll be richer for it.

http://www.desiringgod.org/resource-library/sermons/praying-in-the-closet-and-in-the-spirit