I'm no expert, but I used to breath, then sight until I attended a swim clinic given by N. DeBoom. She said we should lift the head slightly (eyes just above the water), sight quickly, then turn to breathe. Made a world of difference.

Ditto on the "I'm no expert" thing, but the sight-then-breathe approach is the most logical, and when you think about it, intuitive way to do this. Depending on the course and what you're using as a sight target, you may only need to lift your head slightly out of the water to do it effectively. Of course, choppier water will necessitate a more exaggerated head lift but for the most part it seems as though the more you make sighting part of your normal stroke pattern (if that makes sense), the better.