Plaster does tend to crumble, and unless you get lucky and hit the lath, a nail or screw may not always hold. Have you tried Command hooks? I've used those, and they've worked well in my applications. If the surface of your plaster is too bumpy, you can sand smooth and paint the areas where the Command hooks will go. I've also used those picture hanging nails (they drive in at a downward angle and have a little backing plate halfway up on the nail). They work well as long as you're careful to not cause the plaster to crumble.