A lot depends on how hard the plaster is. Plasters are broadly divided into alpha (hard) and beta (soft) plus a range of hybrids of the two.
Softer plasters (especially while still a bit damp) can easily be carved with knives and woodwork type chisels (but don't use expensive fine quality ones).
Microplane/surform rasps are very effective on plaster, especially for roughing out shapes as they are less prone to clogging than other abrasive files.
Single cut rotary burrs can also be useful.
Harder plasters, when thoroughly dry can be worked with most abrasives and will take finer detail. It is also possible to seal the surface when it is close to the final form with water based lacquers, varnishes or shellac to get more of a polish.
The main issue is that any plaster creates fine dust which tends to clog tools quite quickly and, as it also absorbs moisture very readily can also make steel tools corrode very quickly.
For rough shaping, drills and saws can also be useful.