Usually the cause of sagging is the cake or filling, not the fondant itself. If you fill the cake the night before and let it settle before you put the fondant on (some people put something heavy but not heavy enough to squash it flat on top, like a book or a ceramic tile), then it shouldn't sag after the fondant is put on.
Sagging is usually caused by the cake settling. Thats why the most expert of bakers will tell you to bake several days in advance, refridgerate your cakes, fill and crumbcoat and allow to rest overnight, and THEN and only then cover with fondant, once the cake has allowed to come up to room temp.
It also helps when you are using a cream or fruit filling, to make a dam of super stiff BC to hold in your filling.
If your cake is multi-tiered, it is also helpful to have a good doweling system, or SPS, or some kind of structural support for your cake. The weight of the cake above can cause your cake to bow and your fondant to sag as well.
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