The mucus at the back of your throat is post nasal drip. Your sinus is dripping and running down into your thoat, picking up bacteria along the way and causeing the mucus to thicken. The best way to treat this is to get your sinus cleared up.
There are tons of ways to do this, but I use "Oxynasal Wash" from Dr. Donsbach, which can be found at http://www.oxygentherapies.com This stuff works instantly and I find it inexpensive. It has been a life saver for me because my PND always caused choking which was quite embarassing.