Neti pot! If you don't have one (and you don't need to buy one), just use a bulb syringe or some type of vessel with a spout. Google neti pot solution recipes and make your own solution (basically salt water). There are pics on Google on how to use a neti pot. It flushes out the sinuses and looses the packed yuck up there. I take a decongestant right before so they work together and I get relief.
DH swears by the neti pot and used to have chronic sinus infections and was on antibiotics quite a bit. Now, he has been infection free for almost 2 years. He uses the neti pot more frequently as he is prone to sinus congestion.
I also second using vapor/steam whether a pot on the stove or from a humidifier. Hot showers work, too.
If you had a cold and thought it went away and is now back, you could be developing an infection. If you get a fever, you may need a doctor's appt. Good luck!