I have three different plants (Dieffenbachia, snake plant, ZZ plant) that are "flopping" and "spreading" so much that I've had to stake them. They seem healthy other than that. I now realize I've been overwatering them for years. what should I do to get them to "stand on their own" again?
You are very correct! This is a typical case of being over-watered. Now what you will have to do is let them dry out very well. Make sure to give them plenty of light from a window, and I would also recommend using wettable sulfur, or garlic oil in the soil, in case any root infections are setting in. Make sure to let the top layer of soil dry out between watering.