This article is from the Triathlon FAQ, by with numerous contributions by James Goddard others.
The best bang for the buck is probably to find a masters group in your
area. For information on masters swimming in the US refer to US
Masters Swimming http://www.usms.org/.
Many people have had tremendous success with Total Immersion
http://www.totalimmersion.net/. The general consensus is that if you
are not already a good swimmer who is just looking for refinement, TI
is a good place to start.