an informative site -
http://www.waycoolinc.com/z3/essentials/fixit/heeltoe/shifting.htm
the hard part is finding a place to practice. a deserted stretch of road is good. the track is challenging because there's a lot of other things going on.
at a track, you can hear the good shifters by the smooth transition from one frquency to another, and by the lack of "jump" in the car during downshifts. easy to explain, easy to understand, hard to master.