I've tried the viva method twice because I can NEVER create a smooth surface. Both times, the cake looked great, but about 20 minutes later, the icing starts to slide around and wrinkles around the middle where the layers touch. What am I doing wrong????! 

Do you let your cake settle after you crumb coat.  It seems you might get some settling of the layers making the buttercream sag.

You are probably not doing anything wrong per se.  Cakes tend to settle as they sit.  Lots of remedies have been proposed here from creating a super stiff icing damn around the edge, to filling the cake, covering in plastic and laying a heavy tile on top overnight.  I guess you just have to find what works. 

I NEVER have that happen...my reasoning is that I use a icing that doesn't not contain liquid (2 pounds powdered sugar, 1 1/2 cups butter, 1 1/2 cups hi ratio shortening and two tablespoons vanilla) so the sugar molecules never really get "wet"...once they do they tend to form a very slow flowing liquid that tends to bulge out of the sides over time.  Keeping cakes chilled also helps minimize this problem.

No, I didn't know you were supposed to do that. How long should I wait?

Hi Jeff. Does your buttercream crust?! I didn't know you could create a crusting buttercream with hi-ratio. I hate using crisco!!!

No, not really…maybe just very slightly after it’s been in the cooler overnight…but certainly not enough for the Viva method…I just use a bench scraper and angled spatula to smooth mine.