I'm by no means an expert but I've read this before on CC and have seen it work. If you roll out an oversized piece of fondant and sort of elevate your cake on somthing like a coffee can (small than the diameter of your cake), the fondant hangs off sort of like a tablecloth and the wrinkles start forming below the bottom of your cake so that you can get it perfectly smooth.
I know, easier said than done. HTH!
Personally I don't elevate the cake. It is too difficult on the sizes and shapes I create. Instead, I roll out very "generous" amount of fondant and fluff and drape like a wedding gown hem........then smooth from the top and fluff and drape some more.
I found when I use a generous amount of fondant and not rolled too thin it works the best for me.
HTH 
I know what you mean. The first cake I made, turned out perfectly! The second one I made, pleated @ the bottom, so I took it off & tried to stretch it... BAD MISTAKE!!! If you make enough fondant, and roll out about 1 1/2- 2 times the size of your cake, you'll never have this problem again. P.S. make sure your fondant isn't too dry... you will tear your fondant where your icing curves over your cake. You can reuse the remaining fondant again.
I'm so glad someone bought this up! I had a terrible time with the Dora the explorer cake I made last night, the fondant pleated and ended up tearing in places. I made some extra 'magic' stars to cover the odd patch here and there and laid coloured strips over a lot of it which helped disguise it. I cannot believe I still haven't got the hang of this!
I don't elevate my cakes. I work with the cake on a foil covered double layer cardboard cake circle directly on the table. The iced cake is well chilled, the fondant rolled out and draped on the cake. If you consider a 10" cake is 4" tall and you double that to account for both sides, I roll my fondant 20" in circumference so I have plenty leeway to work with. Once it is on the cake I pull the fondant out from the sides as far as it will go without putting stress on the edges of the cake, Do this all the way around the cake, it looks like a full skirt spread out. Now I lift one small section of the fondant up from the outside edge off the table with one hand and smooth the fondant from the cake edge down to the base with the palm of my other hand. Do that all the way around the cake. Then smooth the top of the cake with a fondant smoother, the edges, then the sides down to the base of the cake. Use a plain pizza cutter or pastry wheel and cut off the fondant near the base, all the way around. Give yourself about 1/8" extra when you cut it off. It can be pushed in neatly with the pastry wheel after the surplus fondant is cut away. And then covered with a piped #2 0r #3 beaded buttercream or royal border. Or you can make a fondant rope to go all the way around the base of the cake.
I struggled for months with those pleats from the surplus fondant. This method works better for me than any I have tried. I never have pleats or creases in the fondant anymore.
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