I've gotten really into Total Immersion swimming (demo video here: http://www.youtube.com/watch?v=rJpFVvho0o4
). It's a technique that was developed for triathletes and other distance swimmers. Basically you create as little resistance as possible as you move your body through the water, which lessens fatigue and enables you to swim longer without exhausting yourself. Before I started I would have to rest every 50 meters, but now I can just keep going and going without feeling like I can't catch my breath. I learned from online videos and reading about it. I'd love to take a lesson to perfect my technique, but not too many places teach it.