If you want to hang something on a wall that is plaster, you need to be extremely careful when putting a nail or hook in place. If you do not put the nail in properly, the plaster can begin to crack or break off into small pieces. However, you can minimise the risk of cracked plaster by using some common household tools --- and having a steady hand.
Determine where you want to hammer in the nail on the wall.
Place the sharp tip of the nail against the wall and hold it steady with your fingers.
Gently tap the flat side of the nail with a hammer until the nail is securely in place. While tapping the nail head, make sure to keep the nail at an angle.
Mark the location where you want to insert the large nail. Place the tip of a drill bit against the mark on an angle.
Begin drilling on an angle into the wall where the mark is located. The drill bit should be going into the plaster slanted upward to create a hole that the nail can go into at an angle.
Continue drilling, holding the drill handle in one hand and using the other hand to gently push on the back of the drill to keep it moving into the wall. You may need to gently slide the drill bit in and out of the hole, all while holding the "Drill" button on the power drill in the "On" position. Move slowly, and keep the drill steady.
Gently blow away any plaster bits and slide the nail into the new hole. You may need to use a hammer to gently tap the nail in place. Hold the nail at the same angle that you did with the drill so the nail goes into the hole in the correct orientation.
If you are hanging heavier objects from a large nail or hook, consider nailing the nail into a section of wall with a stud behind it. Use a stud finder, which will indicate where the studs are located behind the plaster. Nailing into a stud will add more stability if you're hanging heavy objects, thus reducing the risk of cracked plaster.
Watch out for flying bits of plaster that can be accidentally inhaled. They are also irritating to the skin and eyes.