These tips are great, all I want to add is to be flexible in the technique you choose. On old properties you are lucky to get a dead straight line to cut in to. I just taped a cornice and got lucky with a great result, but because the main lath and plaster wall was so wobbly, when I taped a line for the picture rail and the skirting boards, it looked like it went in and out like a donkeys doo dah, so I "straightened them out" to the human eye by re-painting the line freehand.