One of my friends had a bunch of flowers that went from dry to total mush when a big thunderstorm came through and rendered her gumpaste humidified. So you want to avoid that.
To store gumpaste, I use plastic shoe boxes that come from the dollar store, or larger plastic bins for bigger flowers. I have them labelled with what kind of flower is in them so that I can go in and grab the right box when I need them.
It's easy to make up a bunch of flowers ahead of time and just store them. If you see photos of some big-name cake designers' studios, they seem to be using the same system, but obviously on a much larger scale. (If I had a bunch of gumpaste servants to do my flowers, I'd have that many bins too.)
What you need to watch out for is humidity. If you don't have an air-conditioned storage area, and a big wave of humidity comes in, gumpaste cam soften up. I sold some gumpaste flowers to a bride to use at her resort location wedding, and it turns out that the location didn't have air conditioning. I sent the flower to the resort, and a few days later they emailed me saying that the flowers seemed to be softening up. I had to have them mail them back to me, and I shipped them back to arrive right before the wedding.
Keeping gumpaste in a totally airtight container can also be risky. The recirculation of moisture in the container might soften the gumpaste too. It's better to have them in something that can allow some air to circulate, which is why the plastic shoe boxes are good. Those things are never airtight, but the lids fit enough to keep dust out.
You also do not need to refrigerate gumpaste. I've had people ask me if they should keep the flowers in the fridge, and the answer to that is no. Room temperature is fine. I put gumpaste flowers in the fridge on cakes all the time, but the danger happens when you take the cake OUT of the fridge. With the difference in temperature, condensation can occur on the surface of the cake, which could affect the gumpaste. It's unlikely, but it can happen.
So keep gumpaste flowers at normal room temperature, low humidity, and let air circulate around them, even if only a little.
Kara Buntin owns A Cake To Remember LLC in Richmond VA, and cake supplies online at www.acaketoremember.com and www.acaketoremember.etsy.com