Give people access to your content on-line, then link heavily to places where the product can be purchased.
Duh.
There are maybe 15 artists whose CDs I'll buy the day they come out, no questions asked. I trust the value I will get.
But I never would have purchased dozens of CDs if I hadn't been able to listen to them first. I would have never spent $25 or so on Against Me! if I hadn't "found" some MP3s online that duly impressed me. Hell, I never would have heard of them! I found Thrash Unreal and Stop! on-line, listened to them non-stop, and went out and bought the whole album. How could I not? Then I went and bought a couple previous EPs too!
Put some free songs on your web sites. Nice bit-rate, no DRM, no registering on the site, nothing. You want 3 million people e-mailing the files and the links to each other. You want those songs on everyone's iPod or whatever. Ever heard of free publicity? Jeez.