This advice was probably the first I’ve ever heard that actually explained when we should show and when we should tell.
Show when the emotion or mood changes. Tell when it doesn’t.
What this means is that, when you need to build up a new emotion or mood in your novel, you need to show how this comes about so the reader is entrenched in the new mood. When you start the next scene in this same mood (and this is a must), it’s acceptable to drop a telling reminder such as, “I still felt like crying,” because the mood has already been established.