Q: How do you know which plants to cut back and how far to cut them back?
A: Mostly from experience, but in general woody (blue mist spireas and potentillas) plants can be trimmed 1/3 off the top and non woody (coneflowers and lilies) can be cut a few inches above the ground.
This is not a hard rule but more of a guideline since there are a lot of exceptions. Always be sure of how to prune a plant before doing it. You can always cut more off, but you can never uncut it.