Boy, do I totally identify with you guys!
Ever since I moved to Southern California, I've experienced constant sinus infections due to allergies. I go between laryngitis and pharyngitis, so sometimes I can only sing high, sometimes only low. Now, I have a big range so I manage to make do, but when I finally got rid of the sinus infection with a round of amoxycillin, I suddenly found I could sing through all of it with no trouble! I was AMAZED. I thought I had lost something since my days at Indiana University, but no, it is the infections. And damn, if it didn't come right back a couple of weeks later.
The fact is, I cannot expect to make a living by singing as long as I live here.
Now, I've got a few tricks that I rely on when I've absolutely got to get better and sing Bach. One of them is guaifenecine, the active ingredient in plain ole' Robatussin (sp?). It puts the moisture back in your tissues, which is why it helps you to expel phlegm, and there's nothing more damaging to your vocal apparatus than a dry cough (except screaming!). When you need to, go see a doctor and hold his knees and cry until he prescribes guai, so that you can pop a pill instead of drinking a triple dose of Robatussin, unless, of course, you like the taste of Robatussin! I'm a total wuss when it comes to yucky tastes....
Drink water ALL DAY LONG. Get a purse or satchel big enough for a water bottle and get in the habit of carrying it EVERYWHERE.
Avoid cashews, chocolate, black or green tea, and wine when you are singing or have to do something big within a couple of days. The aforementioned beverages pickle your tissues, like your fingers get pickled by water after you've washed dishes. The foods are just irritating to the throat, not that big a deal, but every little bit helps.
Coffee, while it does have enough tannin to be mildly pickling, is still great - the caffeine opens up your bronchs like you wouldn't believe! My voice teacher informed me that they used to give caffeine shots to premies. In the womb, the last thing to get "cooked" completely is our lungs.
Rest and exercise both do wonders to keep the muscles of your frame in decent shape, which helps you to relax when singing. A strong body obeys, a weak body commands.
DON'T USE OVER-THE-COUNTER NASAL SPRAYS!!! They mess you up BIGTIME and can be addictive. This obviously doesn't include saline sprays (duh).
I have been using Claritin and Nasonex, but they definitely have a negative effect on my voice. I need to explore alternatives, but I have to wait for my insurance to kick in before I see a specialist. I'll let you all know how the next medical adventure pans out, but I don't recommend either of these prescriptions.
Don't know if any of this helps, but I hope someone benefits from the years it took me to discover some of this!