How should I level (flatten) ceiling joists for drywall?
The way we do it is to strap the joists perpendicular with 1X3 cheap strapping.  Find the lowest point in the existing field of joists and that will be your level reference. We use a laser 360 degree level , but you can get by with a good 6 foot standard bubble level. Mark off the joists in 16" on center with an extra piece of strapping along each wall edge. Then set the first piece of strapping closest to the lowest point of the ceiling.  Use simple shims under the strapping as you go to keep each piece of strapping level side to side and row or strapping to row of strapping.  Secure the strapping with 4d or 6d ring nails to the joists.  It will take a little time, but you should get the hang of it very quickly.  The result should be a nice flat and level ceiling. I know this is a quick description, but I will be happy to give you more info and hints if you decide to use this method. BTW, this is definitely a 2 or 3 person job, and an air nailer will really help.