Are you sure it's sinus? There is a condition called "silent acid reflux" which can masquerade as sinus/allergies/asthma. It's called "silent" because you don't get heartburn. I know, because I have it, and it took years for the doctors to figure it out. Actually, they didn't; I read an article about acid reflux and I had all of the symptoms except heartburn. So I went in and had tests done and they said I had a mild case of acid reflux.
If you find that your problem seems to be worse after meals or drinking certain kinds of soft drinks (like Coke--I can no longer drink that, and I used to be an addict), you may very well have acid reflux. If you find yourself clearing your thoat a lot because it is clogged with mucus, or coughing, it is probably acid reflux.
It's very important to get this treated because if it continues, it can lead to scarring of your esophagus and an increased risk of cancer. Not to mention the pressure on your diaphragm from continual coughing and throat-clearing, especially if you are a woman, could lead to bladder control problems. Mine was so bad (and they told me this was a mild case!) that I would often end up involuntarily urinating and throwing up on myself, which is why I kept going to the doctors, only to be told it was sinus, asthma, etc. Believe me, I was desperate! Now that I am on medication and have learned to modify my diet, I don't have as many problems.