"sagging fondant" can also be caused by not covering the cake properly. It's a good idea to elevate your cake so that it doesn't sit flush to your work surface. If I'm covering an eight inch cake I will sit it on top of a 6 inch cake pan. The fondant hangs down instead of bunching and I can be certain that I can trim it exactly to the bottom of my cake. I had this problem with fondant sagging on my cake until I started doing it this way. When you apply fondant and the cake is flat against the table, the fondant can "bunch up" a bit and even if it's only a fraction of an inch it can be enough to cause your cake to have that "sagging" look.
Also, like Cat said ALWAYS cover a cake at room temperature. NEVER cover a frozen, partially frozen or cake that is too COLD. You're just asking for trouble when ya do this.