Echinacea is very helpful for colds.
The first thing I'd say is, well, don't get sick in the first place (I know this doesn't help you much now, but I'll be brief with a few things). I've been performing on stage for a number of years (as an actor, not a magician), and essentially, your body is your instrument, and your voice especially a huge part of that instrument. From about a month out from a performance I'll start to restrict what I consume. I'll only drink water for the month leading up to the performance, upwards of 2L per day. No coffee, no soft drinks, fast food, etc. Very little chocolate and ice cream (none in the last two weeks). Take special care of weather when going out, bring coats jumpers etc. Even down to never being lazy enough to not bother wearing socks (running on stage with blisters is painful =/). Eat well and sleep well are obvious ones. I'll still go to the gym but I'll ease up a little bit with weights etc.
So if you get sick anyway, which does and will inevitably happen once in a blue moon... Pretty much just drink even more, get a lot of sleep, etc. It may be worth seeing the doctor and telling him that you have a gig in a few days, a deadline you have to be in performable condition for as well. Maybe not for a cold, but in some circumstances you could get something extra in addition to "sleep well drink more". Vitamins are good but obviously make sure they're only supplementing your diet. Plan what foods to eat depending on what you have also (if you need more sugar, more energy in general, etc). Stay active but don't push it also... I had a shocking case of my voice failing the night before a musical... I don't think I've ever drunk so much water in one day in my life.
or go under a ladder,
