Thanks Suki
I have the same problem after flying with a cold a few weeks ago. My sinuses still seem to be giving me problems. Plus I'm suffering again with a cold now.
I'll give the echinacea a go, and like you suggest, have a break from it every now and again. Hope it'll help.
I think also maybe trying to avoid trips to frozen places during the winter months might help ... it's no fun opening the doors to be hit by a blast of minus whatever degrees after a warm and cozy 4hour flight.
Cough!