There've been several topics on this - the heat & humidity are just everywhere it seems!!
Indydebi's recipe is very good in humid conditions and uses standard grocery store ingredients:
http://cakecentral.com/recipes/6992/indydebis-crisco-based-buttercream-icing
I use sugarshack's recipe which calls for hi-ratio shortening. If you can't get hi-ratio, go with indydebi's above.
http://cakecentral.com/recipes/5163/sugarshacks-icing-and-tips
I have great luck with Indydebi's recipe. I live in South TX so heat and humidity is an issue here from April-November 
Indydebi's is "hands down" my absolute favorite. It holds up to extreme humidity and 100 + degree temperatures and it taste really good as well.
Mamawrobin,
Yesterday's cake was iced and it was so smooth with the Indydebi recipe! I had actually torted the cakes and filled the night before. Left out on the table in the cake box. I was so happy for a change! I was ahead of the game. LOL! I iced all three tiers....placed in the cake box and left out on the table for about 3 hours before time to leave for delivery. I only have to drive about 30 minutes or so...possibly 4 hours time has elapsed. I get to the venue and the icing is sliding off.
The venue is FULL of people and wedding starts here in the same room in one hour! So no time to take the icing off and start over. So I patch and cover the best I can.
My question to myself and now I am posing to you and Indydebi....should I have let the car run longer before I took off for the delivery? I think I read here that you don't refrigerate??
Thanks for listening.
Jannie .... did the icing crust well when it was on the cakes? A crusted icing shouldn't slide off. the crusting is what I think of as "the icing as set". I'm assuming there is no refrigeration involved anywhere in this scenario?
I've delivered further than 30 minutes away with no melting/sliding. Many times, the cake is in the delivery van along with hot containers of food, so the temp of the delivery space is up a little, regardless of whether I'm running A/C or not.
Indydebi, yes...the icing was crusted well on the cake. It was so smooth...I was so happy!! No refrigeration was involved anywhere in my scenario. I had made the double of your recipe and made it the day before. I had filled and crumb coated the night before. I don't normally crumb coat at all. I actually had never even filled with buttercream the night before. I was trying to be on top of this wedding cake and be on target. In my limited experience....I have torted and filled the same day as well as put my buttercream on with my tip 789. Do you crumbcoat?
So....since it was crusted and smoothed well...the only thing I could come up with was that the truck was too warm.
question for indy.....have u used hi-ratio in your buttercream? I want to try your buttercream for a cake this weekend.
No, I never have. No particular reason except I believe in "It's ain't broke ..... don't fix it."
Some folks have used the hi-ratio, and I'll ask them to pop in here to share how they used it (qty) and how it worked for them. Good results, from what I hear.
I've used hi ratio only once. I prefer Crisco in this recipe. Like Indydebi said "if it ain't broke". HOWEVER in defense of the hi ratio....I did read in a thread the other day that if you use the hi ratio you should cut the amount of shortening by 1/3 cup per batch. I used the full 1 1/3 cup when I made mine and it was a little "heavy". If you're going to use hi ratio cut it by 1/3 cup (per batch) and it should be fine.
To the op... 
I just don't know. Did you use all shortening or did you use some butter? I am so sorry that this happened to you. But I am happy to hear that you were able to get your icing so smooth. Hopefully this will never happen to you again. I've used this icing so many times and even in extreme heat I didn't have this issue.
Was your cake frozen, partially frozen or really cold when you crumbcoated or iced it?
Mamawrobin,
Heavy as in....too heavy in feel for the cake?
Last weekend, I had the same problem with the icing sliding off and don't know why! So frustrating 
I use Hi-Ratio with Indydebi's recipe and have wonderful results, and up until last weekend, NEVER had it fall off the sides of the cake! I'm in Louisiana and the humidity is so thick, you can cut it with a knife!
I've recently started whipping the shortening and heating the milk...would that have anything to do with it?
I have two cakes this weekend and plan on cutting the hi-ratio by 1/3 cup per batch, as mamawrobin suggests, and will let you all know the results. I also plan on completing the cake the day before, but I'll bring extra icing with me on both deliveries in case I have to do last minute patching! If I have any disasters this weekend, I think I'm hanging up the apron 
Indydebi's BC is absolutely the best tasting...a lot of my family members don't like icing, but they LOVE this stuff!
Mamawrobin,
Heavy as in....too heavy in feel for the cake?
If you're talking about the hi ratio...just heavy...you know how Indydebi's icing is kind of "fluffy" and "airy"...well I used the 1 1/3 cups of hi ratio just like I would Crisco and I should have used only 1 cup of hi ratio...it was ...maybe "dense" is the word I'm looking for ..... 
Maybe not...I'm not very good at descriptive words. Kind of like the difference in a meringue buttercream and a crusting buttercream...You know how the meringue buttercreams are "light" compared to the crusting buttercreams that are a little "heavier"....am I making any sense whatsoever???
It was still a good icing and I may try it again using the hi ratio and cut it by 1/3 as others have suggested. I know that I won't use the hi ratio again without making some changes because I like my crisco version much better. 
Oops...I posted to another thread and thought it was this one!
Well after reducing the shortening and using milk in Indydebi's recipe, the icing is still sagging/drooping but worse on the chocolate cake than on the white cake.
I've been whipping the Sweetex and heating the milk - do you think that's the problem?
I think I'm going to experiment with the following next:
1. Sugarshack's icing - made it once and it's a dream to spread, but I thought it was too sweet and I could taste the shortening in it...suppose a little popcorn salt might help
2. Use Crisco only in Indydebi's icing - I switched from Crisco b/c my family liked Sweetex better but they'll just have to deal!
3. I bought some "icing base" at the local party/cake supply store and you mix it with shortening, milk and PS
So I will give those three a try on my next few cakes and see how it goes! Thanks for the tips!
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