The time to seek professional help is when you begin to strain your voice to the point of losing it.

My allergies make me hoarse sometimes. Right now, as a fer instance. What I have found that works is lots and lots of hot tea (with honey and lemon) and water, trying to keep from clearing my throat too often, and licorice root. You can buy the raw root (it looks just like a little twig, tastes not all that great, and feels like you're chewing in a tree) at many health food places that specialize in raw herbs. There's one around here called "Smile Herb Shop" in College Park, MD.

One of my voice teachers used to swear by certain allergy medicines, and when the worst came to the worst, she had a Doc who would prescribe a steroid spray she would spray down the back of her throat. I personally would not do this except if in an extreme situation, but she said it worked for her those times when "the show had to go on" and she was not able to sing.

Lastly, talk and sing as little as you can between one gig and the next, and warm up before actually going on.