Whenever I cover a normal round/ square (not sculptered) cake with fondant, it looks nice and smooth for a while, and then it starts sagging where it meets the cake-board!! It is frustrating me endlessly!!! 
PLEASE give me some tips - maybe my buttercream is to soft underneath / to much / to little? or my fondant rolled out to thin / to thick?? Or What???!
I am smoothing my cakes down from the top to the bottom to make sure that there are no airbubbles caught under it....
Can it be that my buttercream is to wet, or not wet enough when I put on the fondant?
Any help will be appreciated! Thanx! 
What kind of fondant are you using? If you are making your own, maybe it's just too soft and needs a little more powdered sugar.
How thick do you roll it? I generally roll mine just over 1/8" thick. Any thicker than that will add weight that just wants to pull itself down, particularly if you're not chilling your cake before covering.
One way of helping with this is to chill your cake until the buttercream is completely firm. Immediately cover with fondant and smooth into place. Then pop back in the fridge, uncovered, for an hour. This causes the fondant to firm up in the shape it's in on the firm buttercream, and it "sets" in place. Then, when you take it out, even after the it comes to room temp and the buttercream gets soft, the fondant stays firm and in shape. It wont sag because it's "set".
How thick do you roll it? I generally roll mine just over 1/8" thick. Any thicker than that will add weight that just wants to pull itself down, particularly if you're not chilling your cake before covering.
One way of helping with this is to chill your cake until the buttercream is completely firm. Immediately cover with fondant and smooth into place. Then pop back in the fridge, uncovered, for an hour. This causes the fondant to firm up in the shape it's in on the firm buttercream, and it "sets" in place. Then, when you take it out, even after the it comes to room temp and the buttercream gets soft, the fondant stays firm and in shape. It wont sag because it's "set".
Thanks!! I don't think I roll it out to thick, but I never refrigerated because I am always afraid that the fondant will "sweat" when it goes back to room temp...?? And I thought you must put it on while the BC is still "wet"...doesn't the fridge dry it? But I am definitely going to try it now!
Sorry - I most probably sound stupid, but I never took any classes in this, so there might be some small "secrets" I haven't discovered yet 
When it comes out of the fridge, it will sweat a little bit, but that helps the fondant stick to the cake.
Are you letting the cake settle? If you're baking and then decorating as soon as it cools, then the cake might be settling and that's why your fondant ends up sagging. Usually if you let it settle overnight before you put the fondant on, it keeps it from sagging for that reason.
When it comes out of the fridge, it will sweat a little bit, but that helps the fondant stick to the cake.
Are you letting the cake settle? If you're baking and then decorating as soon as it cools, then the cake might be settling and that's why your fondant ends up sagging. Usually if you let it settle overnight before you put the fondant on, it keeps it from sagging for that reason.
I never decorate the same day, I always let it settle overnight. But I put BC on and then immediately the fondant....I think that may be my problem....I'm definitely going to try the refrigerating next time around and see if it makes a difference.
Yes, chilling the cake before applying the fondant is good advice. I stick my cakes in the freezer for a good 30 minutes or so, until the outside of the cake is firm as a rock. This allows me lots of flexibility when smoothing the fondant without worrying out squishing the cake.
You also may want to try trimming the fondant a little shorter around the bottom of the cake. Rather than trimming it right to the board where even the smallest amount of sagging will be noticeable, leave a 1/8-inch gap between the fondant and the board.
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