How can I repair damaged plaster when I come to repaint?
Here are some general guidelines. This sort of work needn't be complex.

Loose paint and plaster must be removed. Repairs made over them are likely to fail.
Securely-bonded paint does not need to be removed.
Glossy surfaces should be scuffed or etched, or should have a bonding primer applied before repairs.
If moisture isn't a common problem, standard joint compound (drywall mud) is a fine material for skimming. It's inexpensive, easy to use, and easy to sand.
If moisture occurs regularly, use a cement-based setting type joint compound or powdered plaster repair mix. This cures rather than simply drying and resists moisture damage much better.
Prime all patched areas to seal the surface in preparation for paint. Ideally, prime the entire wall for best uniformity.