I acted as a labourer for the plaster I hired to replace my hall ceiling - 3 sheets of plasterboard.
He first applied a basecoat and tried to achieve the best level rather than the best finish. After a few hours he scratched it to obtain a key for the finishing coat. Next day he applied the finish skim coat and went for a smooth finish - when it was nearly dry - after an hour or so as I recall - he slightly wetted it and used his trowel to "polish" the surface and fill any remaining hollows and air-bubble holes.
He only mixed as much as he could use before it went off (actually I did the mixing) - quite a big bucket with perhaps half a bag of plaster at a time.
Where he needed to palster onto brickwork he used a 4:1 mix of Water:Unibond PVA glue as a waterproof membrane - but NOT on the plasterboard only on brick.
This link might be useful...
http://www.diydoctor.org.uk/projects/plastering21.htm
Good luck...
Peccavi
January 2010