Here are the various distillations of Tim Keller's series on preaching called Preaching to the Heart.
Most familiar with Keller on this topic will think of the 6 part series he gave at Gordon-Conwell. GC only sells CD's, so if you are interested in listening to them, you'll have to wait for them to ship it (not to mention having to pay for it *gasp*). Luke Simmons, a church planter in the Phoenix area says they are so good they would be worth 10x the cost (currently $25). Another church planter, Pete Williamson, agrees that paying for them in a world of free messages is still worth it.
For those not willing to spend money, there is a shorter, two-message version available for free. If that littler series whets your appetite, then I've been told that all of these messages are based on the D.Min class, Preaching Christ in a Post-Modern World, that Keller taught with Edmund Clowney, over at Covenant. Here is the 189pg syllabus for the class
It appears the syllabus is actually for a class Keller taught at RTS and not at Covenant. While I'm sure there will be much in over-lapping content, the syllabus may not match up with the lectures exactly..
I found a source required by Keller on page 12 of the syllabus: Keller, Tim; "Preaching to the Secular Mind" The Journal of Biblical Counseling 14:1 (Fall 1995), 54-62.