I find that something lasting longer than just a few days, and lasting around a week, is usually indictative of a sinus infection for me. Sometimes I won't ever see green or yellow mucos, only sinus headaches, sore throat, etc, lasting about a week. Generally my symptoms *really* worsen over the course of the week, and sometimes I will cough and become short of breath. Keep in mind, though, that I am asmathic, so you might not get the last symptoms I've mentioned. All these symptoms are fresh in my mind, so I just had an infection last week.

I am also on Flonase, and find it helps me!