So when I first cover a cake with fondant, I can get it really smooth and nice looking, but a few hours later it like sags and ripples at the bottom and looks like crap! How can I prevent this from happening???

Is it butter cream or ganache applied on yr cake. has it crusted? are you using home made fondant or store bought? May be you didnt smooth it out enough. I have a strong feeling its something to do with the icing under the fondant not having set enough, before you applied the fondant..

I know this response is way late! LOL But thanks for your advice! I think it was a combo of me using too much icing and not letting it sit long enough before covering it with fondant!! Thanks again for your response!