Please look at my cake and tell me how I can prevent the sag in my buttercream. It is very frustrating, and happens in most of my cakes. 
URL: http://www.cakecentral.com/cake-photo_1337455.html
when you said "sag", I expected to see the icing sliding down the side of the cake and I dont' see that, so I'm not sure what you're referring to, unless the picture doesnt' show it very well. The closest thing I can see is on the 2nd tier down, there appears to be some roughness just above the ribbon.
Are you using the hot-wet knife method to smooth this? Is it a crusting icing or a wet icing?
It does look like the 3rd tier down needed a bit more leveling (it slopes downward on the right side), and it's not quite centered (closer to the right edge than the left edge). However, it appears you did a good job on getting the dowels all the same size .... with a sloped cake like that, the cake has the potential of leaning, but it doesn't, which tells me you cut the dowels nice and level.
I really REALLY like the vining effect on the corners of the cakes, though! Those look really nice!
Indidebi, yes on the second tier and the bottom tier the icing sagged on me. I use a buttercream with shortening and butter and the viva method for smoothing. The cake did shift on me a bit after it was placed so it is off center a bit on the third tier.
All your cakes are beautiful! Just wanted to suggest experimenting with a different buttercream recipe. Having lived in Arkansas (now in Florida), I know the humidity is high, there. I have had so many flops this last month, due to humidity, I get sick of the cake before I even begin actually decorating. I have now found a great fondant recipe that seems to be working well. I am going to try Sugarshack's buttercream as soon as I find the hi ratio shortening. Good luck!
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