Why don’t you try making some sound while exhaling underwater. Like “booo” or something. It helped with me. And you can try doing it as an exercise first – stay on the edge of the pool and do sets of underwater exhalation with the sound. Try to empty all your lungs before you go out of the water. After doing the exercise you can try “sound exhalation” with the crawl stroke. ;)

You may just be taking too many breaths? Are you breathing every two strokes or less often than that? I go for three or four depending on the point that I’m at in the workout; at the beginning of the workout, you simply don’t need to be taking as many breaths as you do later on. Plus, this will keep you from cramping up (if you have that problem) later in the workout.