the cake showing through the icing in some spots, without making it very, very thick? I have a buttercream wedding cake to do in two weeks, but I have been practicing, but there are always spots where I can see cake.

You have to put it on thick enough that the cake doesn't show through.

If your cake edges are crusty, cut it off. If other portions aren't even and show through after scraping the sides, you'll need to trim them off too or cover it with more icing.

Do you crumb coat?

Coat the cake with a really thin layer of icing first then let it set before doing the final coat. The final coat doesn't have to be very thick and you won't have to worry about cake showing through.

Also, make sure your layers are very flat on top and sides before assembling and icing.

When I'm doing square cakes, I almost always coat them with chocolate ganache (rather than crumb coat). It serves the same purpose as crumb coating and sets up to be rigid, perfectly holding the shape of the edges and corners. Then I lightly ice with buttercream and apply fondant. There are lots of posts in the forum archives on how to make and use ganache. I don't know what I'd do without it!

Thanks for the tips. I will try trimming the edges. Fortunately it will be white cake. How does the ganache hold up in warm weather?

The ganache gets less hard when it warms up, but still holds its shape better than buttercream. It kind of "soaks" into the very outer layer of the cake, so there's no letting go and sliding off the cake.