Here Kirin,
This should help you!! http://www.cakecentral.com/article83-Quick--Easy-Smooth-Icing-Using-a-Roller-Melvira-Method.html
OOps!! Gave you the wrong technique. This method can be used with the Viva papertowel too!! I put the viva papertowel over the crusted buttercream and smooth in gentle circles with my hand. Some on CC do this as a first step, then use the roller over the papertowel and finally use the roller over copy/printer paper.
When you take the wilton course they teach you to smooth your cake with a spatula as smooth as you can get then let it sit about 10 minutes or so. The frosting will crust(harden) then you take the smooth side of a paper towel and start to rub the frosting not pressing. If you press the frosting will stick to the paper towel. And you keep lifting the towel and moving it rubbing allover the cake until it is smooth. I'm not the best at explaining!
place paper towel over crusted buttercream (smooth side touching cake) and rub the towel gently in circles without pressing. Once you see how this works, you can assess how much pressure to use. I like this method, it works better for me than with wax paper.
) VIVA ROCKS. I have attempted several times to do this in the past and could not get it to work. I then attempted it in my Wilton Class and it worked wonderful and haven't had a problem since. It may take a little practice at first but once you get it you will think it ROCKS!
Viva is great and gives a smooth finish - but try experimenting with textured paper towels too - I get some of the greatest oohs and aahs over the textured pattern left by some - it can be a really neat effect. I use a plastic putty spatula for smoothing the paper towel - I think my hands are too warm sometimes and then the frosting sticks, and that's just a pain! I was so glad to find the paper towel method after years of the hot spatula that took forever to get the frosting smooth and nice looking! Viva the Viva!! 
Yes, I have used the high density foam roller method and I love it as well as the Viva method. I just followed the directions and it works great. At my Wilton class they asked how I got my frosting so smooth. When I said that I used the high density foam roller they looked at me like I was crazy!
This should be a link to sugarshacks buttercreamrecipe & method for smooth frosting. Here, she says she uses her hands on the viva to smooth the icing after it has crusted..... The warmth of your hands helps it melt slightly http://forum.cakecentral.com/m.....c&&start=0
good Luck!!!
I bought my roller at Lowes and it was under $6.oo dollars. It is small and I was afraid that it wouldn't work-but it does a great job. When you are finished smoothing your cake just wash the roller with soap and water and let dry. I also purchased a bag of 3 extra rolling pads so that I could have a dry one if I needed it.
I use the Viva too. I use my hand then a fondant smoother to get my sides smoothe AND straight. I also like the rollers, which I got at Wal Mart. For tiered cakes it's helpful when smoothing the sides if you can put your cake on a raised surface, turntable is best, so that you can smoothe all the way down to the bottom edge of your cake. Just like you'd do with fondant to trim the excess. It doesn't take long to get the feel for smoothing with the Viva. Just make sure you let your icing become dry to the touch so it won't stick to the paper towel or rollers. If it does stick in a spot you can dust the spot with powdered sugar and it won't stick. * Once in a while it'll stick in one little spot for me.
I used the Vival method until I got my roller. It's amazing!!!
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