I recently had to ask this questions myself. I would have to say the viva paper towel method works well, the only think I personlly don't like is the icing doesn't look quiet as crisp as it does before you do it. I did find making my scrapper warm under hot water and completely drying it off did help before even doing the viva paper towel. Best of luck!!! I haven't done the roller method yet but have it in my stack of must do's.
I had a difficult time getting the tops of my cakes smooth, then I saw the article for the upside down icing technique. I love it! Works great for me. I use a combo of that technique and the Viva papertowel method. The link for the upside down icing technique is below. HTH.
http://www.cakecentral.com/article6-Upside-Down-Icing-Technique-for-Perfectly-Smooth-Icing.html
I use a big metal scraper from the hardware store. I can usally smoothe the top with one or two sweeps.
Sugarshack's DVD!!!! Amen, Amen, Amen!!! Works everytime and my frustration level when attempting smooth BC has gone way, way down--If only I'd had her video 5 years ago when I started this!!! Ditto on what becklynn says--so worth the $$$$.
Totally this. I get so many compliments on my smooth icing, thanks to Sugarshack 
. I also use Melvira's method when necessary.
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