How to prevent sagging ceilings when joists are spaced 24"
The simplest thing to do is install strapping - 1&quot; x 3&quot; furring strips that run perpendicular to the joists.  This makes drywall installation a lot easier, gives you a wider surface for joints, and lets you use thinner cheaper easier-to-handle drywall without it sagging.
Strapping is used a lot in some areas of the US and seldom seen in others.  I was skeptical about it, seemed like just wasted material, but when you consider the advantages and the low cost and effort needed to install it, it seems like a pretty good buy.