Frost your cake using a crusting buttercream (faux fondant recipe on this site). You can try the Viva Method. Personally, I can't say enough about the Melvira Method (also found on this site) You buy a high density foam roller from Home Depot. Let your buttercream crust for about 15 minutes. THen roll over it with the foam roller! It couldn't be easier!
Use the Viva paper towel method. Try searching for this on this site. I use it all the time and I love how it makes my cakes look. I have yet to work with fondant so this is the next best thing. Ice you cake like you normally would and let sit for about 15 minutes. Take the paper towel and lay over cake and use your hands or a fondant smoother and lightly rub over the cake. You will see the icing smooth out. You can do this to the sides of the cake also.
Thanks
Jen
I have to second the Melvira Method. If you want to see an upclose of the results here is a link to the photo of the first cake I used this technique on. Everyone couldn't believe it wasn't fondant lol
http://www.freewebs.com/ohmygoodiesco/010107_2.jpg
You don't want to press too hard because you'll dent the icing and it'll also remove the icing. But if you let it crust first you should do just fine. I picked up my roller at Lowe's home improvement store for under $5.00. 
Melvira it... no other way but melvira in my house!
Melvia method all the way ..... Melvira method only! I first used it on my M&M cake (in pics) and have used it on every cake I've made ever since. The Govornor's Award cake was a 22x28 and you can imagine how difficult it is normally to smooth that much surface!
I'm tellin' ya ..... once you try this method, you will NEVER use any other method. I promise you!!!
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