Don't make the tea too hot, though, and don't put ice in your water. Extremes of hot and cold aren't good, either. A steamy shower before your gig can help moisten the sinuses and loosen any mucus. Taking a cue from Sting, you might also want to invest in a humidifier-not for the stage but for your house, especially during the winter, when household heat dries out the air. Leave it on in the bedroom during the night while you're sleeping.
Antihistamines can also be very drying.
If you need them, ask your doctor to recommend a type that doesn't have dry mouth as a side effect. It's amazing how much proper posture can contribute to your singing. On the other hand, don't overdo it and arch your back. To get the right posture, lift your shoulders, move them back without overextending, and let them drop. Some people find that practicing yoga or the Alexander technique help greatly with this.