After years of making cakes, I’ve only just learned about using crumb coats – a thin layer of icing to trap the crumbs – to make the task of icing the cake easier. Another handy trick is to freeze the cake while it is still slightly warm. This inexplicably makes the final (thawed and frosted) cake incredibly moist. Some people suggest that it “traps the moisture in.” Maybe. Heh. Maybe. It also lets you wait indefinitely before decorating your cake. Combined, the techniques are even more powerful: the thin crumb coat freezes hard on the frozen cake, keeping those crumbs good and stuck and letting you glide over it like a frozen lake when giving it the final coat.
To make a crumb coat, all you need to do is spread your frosting over your cake very thinly, not really caring about tearing up crumbs. Let the cake sit for about 15 minutes or so, to let the frosting layer cure and harden, trapping all those crumbs forever in their carbonite-like tomb. Just use the same frosting that you plan on using for the outer coat, so there is no need to have different batches of frosting (unless this is something you really want). Then, after the frosting has cured, put on your final coat of frosting as you would normally. Note that you no longer have any problems with pesky crumbs.
Sure, you could be extra careful and use your mysterious tricks to not get any crumbs in the first layer and call it a day, but honestly I can rarely get that to work. Plus it is so much easier to frost a cold cake.
Some people say they get mixed results with freezing, and also suggest not to crumb coat a frozen cake (they suggest letting it thaw first), because their frosting separates. I use buttercream frosting and I have never had problems with it separating on a frozen cake, but I suppose that is something to keep in mind.
If only I knew it sooner, this trick could have saved me so many tears!