Caruso et al's old remedy was to slice up an onion, cover it in brown sugar and leave it overnight with a damp cloth over it. Drink the brown stuff the next morning (it's not as foul as it sounds). Basically Covonia, but has worked for me. Also might be worth checking your singing technique - try to vocalise using just your diaphragm and nasal passages, rather than singing from the throat and chest.