I saw this the other day... http://www.cakeartpartystore.com/store/e-StoreOperations_viewItem.asp?idProduct=22376
I don't know if anyone's used it, or how well it does, but it might be worth looking into, or asking about. Maybe someone here has used it and knows if it's worth buying or not.
Sorry I couldn't be more help.
Good luck on your display cakes, and your business!
Holly
I am a wilton instructor and I am all the time doing displays for Michaels. I find that the fondant fades VERY QUICKLY, like within the first day! So I had recently started a thread asking if anyone could give any advice if I could spray it with laquor or polyurithane to keep it from fading. Im currently working on a display and I plan to spray it just to see if it will work. Mine will be only fondant but I do have some with gumpaste flowers (which totally fade!!) and RI and I plan to spray those as well. That will be sometime next week. But the displays will hold up for a LONG time. I did the tea pot cake I am holding in my avatar and its been up for almost a year. Looked horrible since it was faded and it was SO DUSTY. I can try and let you know when I spray them unless anyone else has any scoop on the polyurithane that I don't know about.
ginger
You could also try using 'Plaster of Paris' or some sort of plaster (if it's not called the same thing in the USA).
You could add realistic plastic fruits, etc for more variety.
If you are spraying it, I would suggest matte finish, otherwise people will wonder why it has that funny shine to it! 
I was going to cover some dummies in joint compound I had sitting in my garage today - but I opened it, and it had mold all over it (seems my ex didn't put the lid on all the way). So I've been thinking about it all day, and I'm thinking about trying grout instead - so that I'll be able to wash it. I'm heading to Lowes in just a few minutes as a matter of fact.
Maybe I'll pick up some grout and some spackling compound and see which works better.
I bought a small container of joint compound - and it's really heavy... so I also picked up a container of "Fast 'n Final" Lightweight Spackling (32 oz.) - but didn't have much hope for it (it's so light, the container felt empty). When I got home, I opened it (the LS) up and it was dry and crumbly, so I mixed it with water until it was the consistency of buttercream.
This small container (which cost less than $5) iced a 6" & 8" dummy with a little left over.
Once it dries, I plan on sanding it and coating it with a matte clearcoat. So far, it looks awesome! It could pass as a fondant dummy in pictures, and if the clearcoat works, I can decorate, wash, decorate, wash, ad infinitum. I'm so excited! 
BrandisBaked, I hope you have better luck than I did. I tried using spackling once, and it dried gray (when it finally dried). It cracked as it dried too.
I'd love to hear from anyone who has used the Perma Ice, I've been considering buying some.
indydebi- Do the displays iced in Crisco sit in a display case or anything? I'd be afraid they would attract flies! This is very intriguing... do you thin it with water to pipe it?
No display case .. on a table top right in the open. No bugs, no flies, no mold, no discoloration. No thinning. I use the same recipe that I use on cakes except I omit the vanilla and dream whip. It dries to a concrete-hard and they last forever.
I do the same thing--regular Crisco bc over the dummies. Mine are on a bookcase and a buffet in my dining room. No glass. Never have a problem with bugs, flies, cracking, crumbling either. And I live on a farm where the fly to cow ratio is a million to one.
I have dummies covered in fondant that have been sitting in my garage in boxes since January (they are boxed), before that they were just sitting in my basement (not in boxes) since February 2006. They look fine. Your colors won't fade as quickly if you use Americolor. The purple diamond cake in my pics is a dummy and it's still that color. That was the 3rd time it was made, the first 2 times with Wilton colors and it faded overnight, and the last time with Americolor and it still looks good. Just took them out the other day. You do have to be cautious of direct sunlight. If it's possible for you to rotate them so they aren't constantly in direct sun, they should last longer.
Well, my windows are south and west facing. Ugh. But it does make for a nice bright shop. I think I'll try the Americolor with the light spackling. Hubby's a finish carpenter. He knows a lot about spackling and can probably give me a hand, plus I just live a few miles from Cakes by Sam. She probably stocks the Americolor. Thanks everyone for the tips. I'm going to get cracking on some display cakes.
Hi there i dont know if you anyone said this yet but there is a product you can buy this is made for display cakes and pipes out like the real deal but holds up like plaster
I posted a link to that very thing earlier, but I don't know if anyone's mentioned actually using it.
I'd be curious to know how good, or bad, it is.
Thanks for bringing it up again, maybe someone who has used it will see this and let us know. 
Holly
I haven't tried the Perma-Ice... it's much more expensive than the spackling. Plus I'd have to pay for shipping.
I'm going to run to Lowes today for more spackling. (I get a 10% military discount too)
My bakery windows are facing west, so the display cakes were fading so fast.
I am using acrylic paint colors now (for demo cakes only!), it works wonderful!
I use Royal icing, it is easy to work with and cake displays last 6-8months, and then I usually have ideas for new display cakes, so it works out great for me!
Quote by @%username% on %date%
%body%