Invest in a good scraper for the sides. Make sure your buttercream is thin enough (for the longest time I couldn't figure out why my BC was giving me a rough stucco look. Turns out, I just need to thin it out a bit).
The process I find that is easiest is: Apply a crumb coat, let it sit in the freezer for 10 minutes. Put on another coat, freeze for 10 min. And then put on a final coat and let it sit out in the open and crust for about 30 minutes. Once it has a good crust, I use a high-density foam roller (http://cakecentral.com/articles/126/quick-easy-smooth-icing-using-a-roller-melvira-method
). Then I go over it again with a Viva paper towel, just to make sure.
And finally...PRACTICE PRACTICE PRACTICE!!