*See post below for a better explanation!!
I learned this in Toba's book and had been faithful about doing it, then had some sort of brain malfunction and hadn't been doing it on my cakes for bridal tastings and was fighting the bulge again. Tonight I just frosted and put fondant on a cake I spackled and lo and behold perfectly straight beautiful sides 
So for those of you out there fighting the bulge, I highly suggest spackling your cakes. All you have to do is mix some of the cake with the buttercream as your crumb coat. Never again will I not spackle!!
Kristin
Oh, sorry!! Ok, you take left over cake, like what you cut off when you leveled it(she suggests part of the filling too but I didn't like the way that worked) and put it in a bowl and kind of chop it up into crumbs and then mix it with your frosting, then use that mixture to frost the cake. It is a lot more solid than just the frosting by itself and holds the fillings in better. Then, of course, you have to put your layer of frosting over that. Is that a much better explanation? I really shouldn't post instructions at night when I am tired 
Kristin
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