I've been restoring the plaster walls in my house, and have good success using a product called DuraBond 90 from CGC.
This is a setting compound which hardens in 90 minutes. It's wonderful stuff, and I now believe that the marketing claims on the box, "will not crack, chip out, or break away", are true.
One advantage of using this stuff is that you don't have to use paper or fiberglass tape, both of which can be hard to conceal -- it's very difficult to use an 8 or 10-inch compound knife to feather a taping joint when the plaster wall has peaks and valleys.
I fixed a large number of cracks and holes in my walls by opening them up by hand or with a Dremel tool and filling them with DuraBond, scraping off all excess. To finish the surface, I've used drywall compound instead of plaster.
When I've had to fill large holes (some of the scratch coat had completely disintegrated underneath trim I had removed for stripping), I would do the following:
1) Clean dust from edges of hole. Make sure a backing material like lathe is present.
2) Cut one or more pieces of thin drywall so that together they "fill" the hole, leaving a 1/4-inch gap around the edge (and in between pieces)
3) Arrange the pieces in the hole, using short drywall screws just to hold them in place against lathe or whatever backing is present. This is just a trial fit.
4) Remove the screws and drywall pieces.
5) Spread DuraBond on the back of each piece of drywall, replace in the hole, and reinsert screws.
6) Use a 2" compound knife to force DuraBond into all cracks and in between the drywall pieces.
7) Use a little more DuraBond to bring repair to a level matching the current scratch coat, at least. If you fell you're putting too much in at once, apply in layers, allowing to dry in between.
8) Let compound dry. (You can remove the screws now if you wish.)
9) Finish with drywall compound (or CGC's drywall 20 or drywall 90 setting compounds) to provide a sandable surface. You could finish with plaster, though I haven't tried this yet.