Now hold on JEri, I've got the monopoly on squeaks around here.....
Seriously, a hip flask full of half and half (or 2/3 to 1/3 if you prefer) of port and brandy. Take sparing sips during the concert and get a cab or someone else to take you home.
I suffer from asthma and have found that after a weekend of singing, or with regular choir practices, my peak flow goes over 400. When I sang regularly with 2 choirs and a folk club, despite the smoky atmosphere of the pub, the peak flow was an average of 750 - 800, double what I can manage now. So keep doing the excercises. Set yourself a choir practice at home, where you just stand and sing things for an hour or two. It will help your breath control, phrasing and memory too. I have to have 1litre of water with me now to get through a church service of an hour and a half, that just about keeps it nice and moist. Oh, some people use asthma inhalers, like ventolin. These have the effect of widening the bronchioles, those little bits in your lungs that look like coral branches. Sometimes it works, others it doesn't. If it wasn't prescribed for you, don't take it. Widening perfectly healthy bronchioles just means you can take in more of the pollutants in the air, like smoke, water mollecules and dust, and only causes greater problems in the future.
Good luck with it, I prefer the port and brandy cure myself, but then, I would!