Repair of old plaster walls can be both time consuming and rewarding. There are several steps that need to be taken before you start.
First and most important is the integrity of the plaster. Depending on the age of your home, the plaster may be; Lath and plaster, blueboard with a plaster topcoat or a screen with a plaster topcoat. Check that the walls and celing are tight and snug to the framing of the house. Using your hands, press on the wall to see if there is any movement. Don't pound on the wall, remember, plaster is brittle.
Secure any loose areas (carefully) with Plaster washers (about the size of a quarter and is screwed in place with a drywall screw).