Hot beverages are excellent for thinning mucus. If the mucus is draining from your sinuses to your throat, you will want to thin out the mucus that you are having to clear your throat with. Zinc losenges such as Cold-Eze (not on an empty stomach) will help do this - it is the solution which many singers use for perfect clarity in vocal recordings when they have excess mucus. Herbal teas such as peppermint will add a menthol like effect and clear your sinuses. If you add the juice of a lemon wedge and add in honey (which will help your loss of voice - and honey is an anti-bacterial/anti-viral added protection) - it will be soothing on the throat. The honey and lemon together (2 minutes in the microwave for 8-10 oz mug), should do the trick. Singers, broadcasters - do this often. Lemon is added vitamin C and will cut through thicker mucus.
For the sinuses - eucalyptus oil aromatherapy is fantastic for opening the sinuses so you can actually breathe. A warm bath with a few drops of eucalpytus oil will help you breathe easier. If you are not into baths, put a few drops on a tissue and inhale. Eucalyptus is an anti-bacterial and anti-viral oil and inhaled - helps the respiratory system. When added to a carrier oil such as jojoba or sweet almond (as you do not want to put the drops directly onto your skin) - put on the temples will help with sinus pressure and sinus flare up headaches.
Above all - you will want to STOP eating dairy products and refined sugars during sinus flare ups. The more dairy you eat - the more mucus the human body produces. Refined sugars tend to aggravate phelgm production, too.
Good luck and feel better!