This is a useful tip from a health advice email subscription.  Re-posting it here, in case you find it interesting, too:

"Swimming – Beginning swimmers often tire quickly because they take too many strokes before breathing. You need plenty of oxygen to fuel muscles while swimming, so practice breathing fully and rhythmically.

• Exhale under water through your nose and mouth before inhaling when you surface.
• Breathe through your mouth every other crawl stroke.
• Keep your breathing relaxed and smooth, not choppy.

Proper breathing is essential for a safe, effective workout. Only by breathing efficiently can swimmers sustain their session to improve their cardiovascular fitness."

I am guilty of taking too many strokes before breathing, but that's probably because I lift my whole head out of the water instead of twisting it, and then start to sink.  I still have to learn the twist-your-head-to-the-side technique.