how do I proceed when I need to adjust down a roof that sags in the middle? is there any good site where I can see how to do it? anyone know? 
A rotating laser, the kind used by ceiling builders, is a good recommendation for getting it straight.
Place a U-profile around the walls like Lindab's SK30 or SK43. In this, hang mexihatts (s-profile - secondary profile) across the room. Hang the plasterboards from the mexihatts.
Voila - a straight ceiling.
http://www.lindab.com/sweden/products/pdf/profile/byggreglar_139.pdf
http://www.lindab.com/sweden/products/pdf/profile/byggreglar_136.pdf
Voila - a straight ceiling.
http://www.lindab.com/sweden/products/pdf/profile/byggreglar_139.pdf
http://www.lindab.com/sweden/products/pdf/profile/byggreglar_136.pdf