best technique for a good finish and straight cutting in edges

Horses for courses for roller and brush choices - I use Purdy and Hamilton sleeves for the rollers and Purdy brushes. Not really an "easy" way to get good lines when cutting in mate other than experience. A good quality brush shaped for cutting in (must be some vids on youtube on how to trim a brush up for this) and loading a brush up correctly all helps - don't forget, the quality of the plastering effects how easy it is to make a line look straight (when doing wall to ceiling). If ceiling is up and down then I find it best to line onto it slightly, difficult to explain. Caulk to skirting and doors before painting - this helps to keep lines good In terms of doorframes / skirting - I use a waterbased undercoat and topcoat - keep the paint warm as you can - put your undercoat on (after the prep of course) and get it on fairly quickly. I then lay it off by dipping the brush in a little water and this tends to do the trick - apply your 2 topcoats and lay off as with the undercoat. Watch the runs on the edges as you paint - do the frames and skirting before the walls.