I know this is very similar to other topics in here but there are loads of different answers so thought I'd give a precise description.
I fixed up curtains to the plaster wall in my flat a couple of weeks ago. The fixings are a hook on which you lay the curtain rod. Each hook has two screws that secure them to the wall - a long one and a shorter one. The longer one had a rawl plug in it.
Unfortunately, the plaster crumbled away from the screw fixings on one hook and the curtain rod just fell down. The fixing on the other end seems okay for now.
My landlady suggested filling the hole with polyfilla and screwing the screw back in with a rawl plug. I have my doubts as to whether this will work as the plaster around the filler will be pretty crumbly. I will try it, but would like to know if there are any more reliable solutions first!