Always remember to exhale on exertion, that is, when you are using the weight to target the muscle / muscle group; breathe out so that you take the exertion.
When doing ab crunches, it is the same thing-you breathe out when you target the muscle on your way up. Squeeze the muscles of the abs and breathe out as you come up, if you are executing the normal crunch. Breathe lightly and in on your way down.