how do keep buttercream icing from harding on cakes the next day

If you are using a crusting buttercream, I don't believe that you can keep it from crusting. If so, maybe someone else might know a trick or two.

If you cover with plastic wrap the icing will stay soft. I found this out by mistake a couple of weeks ago....I iced an 8" double layer cake that I didn't need. I wrapped it loosely in plastic wrap so that the plastic wouldn't mess up the icing and when I took off the plastic wrap it was soft......like before it crusted. This was Indydebi's buttercream btw. As long as you keep air from it it won't "crust". Like when you have it in a bowl and to keep it from crusting over you cover the bowl with plastic wrap......works just the same after it's put on a cake. The icing on my 8" cake had already crusted when I covered it with the plastic wrap....but when I removed the wrap a couple of days later to use the cake it was soft. I had to let it "air dry" to crust again.

So.....try wrapping in plastic wrap.

Good idea, I gotta try it.