There are two common causes of "post-nasal drip". As Richard mentioned, allergies can cause mucus to run down the back of your nose into your throat. If allergies are the cause of your symptoms you will likely have a runny nose and congestion as well.
The other common cause of excess mucus in the throat is acid reflux. Reflux can create areas of irritation in the throat that your body creates extra mucus to protect. If you don't have allergy symptoms, the excess mucus your experiencing is probably from acid reflux.