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.