Any Ideas on How to Get Rid of a Persistant Cough?

I am one of those people that whenever I catch a cold or allergies come around, I get a cough that will not go away. I have been to the Dr. about this on occasion and he cannot find anything wrong. I am thinking of going again, because the constant coughing is making me lose my voice and my throat hurts. Plus, it is embarrassing to always have people know your coming around the corner because they can hear you, or you have to excuse yourself on the phone several times for coughing in some one's ear. The cough I have now, I have had since before Christmas.

Any tips on getting rid of a cough? In case you didn't know I am taking medication for High Blood Pressure and some medicines may react. Help me Please!