If you have an old house with holes in the plaster, here's a quick way to patch them. Apply your base coat (or brown coat) of rough plaster with a small trowel. If there's no existing wire mesh, make sure you staple some on first for support. Smooth the plaster out with a large trowel and let the rough coat set for one to two hours, depending on the humidity. Then apply a finishing coat of premixed joint compound. Let it dry overnight and with a 12-inch knife, apply your final skim coat.
For more on plaster, consider:
Patching Plaster Walls
How To: Rebuild a Wall with Three-Coat Plaster
Blueboard and Veneer Plaster Offer Old-Style Look