Pop a line a few inches down from the cieling all around the room, take a rotozip with a 3/4 inch router bit along the line, you may need several passes and it helps to screw a guide to the wall. Then fit a 1x in the slot using a straight line, glue and very slim shims to create a straight platform for nailing crown or cieling trim.
Works for windows and doors and base trim also.
Bob