Re: installing crown molding
Essentially, your moulding needs some moulding.
Once the crown mould is on the wall, measure the point where the gap is largest, between the crown mould and the ceiling. Next measure the length of the crown mould.
Get a piece of 1x material, cut it to the same length as the crown mould, and draw a cut line on the 1x to correspond to the curve in the ceiling. Cut on the line and you should have one straight edge and one curved edge, on the piece you just cut. Hold it up against the curve in the ceiling and see how it fits.
You'll have to fool around with the reveal where the crown mold's flat surface is to make the moulding look good. If it shows a quarter inch of flat space, you'll want the same quarter inch showing all along the crown mould and the curve will have to be whatever it is for the situation.
Hope that helps a little bit.