How do I prevent this sagging of my cake or mmf. I really can't decide what is causing it. I thought maybe it was the cake itself. I thought maybe I was not "resting" the cake long enough. I let this one sit overnight before covering it and it still happened. Is my mmf too soft maybe? I find that when I roll it out thin it tears and shows every flaw in the cake, so this time I went thicker.....same sagging result. Anyone have any advice?
I'm looking but not seeing the sagging you're talking about. 
I've seen far far worse!
OK is the 'box' part of the cake torted and filling with filling? Maybe you over filled. Is the blue fondant totally covering the box or is it just up the sides? If it's just up the sides maybe that's the problem. Maybe if you covered totally in blue and then added the black fondant for the box top, maybe that would help.
The blue is totally covering the cake. The "top" of the lid is painted on and then I added black strips of fondant for the sides of the box top. The cake is torted and filled with buttercream. I don't think I added to much. I usually wish I had added more.
Ok, I'm no expert, but I have found four things that really help when covering a cake in fondant, in order to get sharp corners and smooth sides. First, make sure the top of your cake is level all the way 'round. Very Important!! Second, Make sure you do not overfill, as said previously. Under the weight of the fondant, the filling will just squish out of the sides, causing bowing or tearing. Third, make sure your sides are smooth and plum. On an episode of Ace of Cakes, they used an exfoliating glove to remove any imperfections - something I now use frequently! Fourth, after you crumb coat, let the cake chill for a minimum of 2 hours (if you have more than three layers of cake in a tier, it may be helpful to even freeze your cake). After all of this, you should be able to roll out your fondant and cover fairly easily. Tips: 1) Work quickly with your fondant, and let the heat of your hands do most of the work! 2) If you need SUPER sharp corners, you can either use the piece by piece method, or use several layers of fondant. Good luck, and if anyone has any more tips, please let me know - I'm always learning! 
The only problem I have with chilling the cake is that after you put the MMF on you get condensation, will eventually go away, but is not a good look or help your fondant. You need the cake to be at room temperature in order for the fondant to stick and not get wet.
I had the same thing happen with the same cake you did! The first time I tried fondant it was PERFECT! Now, I don't know what's happening! Take a look in my photos at the shoe cake...I have never had anything THAT lumpy and the fondant was tearing the moment I put it on!
Thanks for your responses. I will keep these tips in mind. Sometimes it is so frustrating to decorate and sometimes it is pure pleasure. ...But, not enough to make me throw in the towel. Thanks for all your support and for all that make this website "work."
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