How should I repair drywall seams that have developed cracks?
Depends on why its cracking..
Were trusses used for the roof construction?  Many problems are caused by wind and truss &quot;uplift&quot; The solution for this problem is a special, movable joint.  No nails or screws are used within 18&quot; of interior wall's top plates.  Special clips are used that allow movement.
Building Sciences Corporation report on truss uplift
The expense and hassle of retrofitting &quot;floating corners&quot; might mean trying a number of fixes.
First might be be using a lightweight elastomeric patch, such as


Product Description:
DAP® ElastoPatch® Flexible Patching Compound is a high performance, elastomeric spackling compound
that eliminates recurring cracks by providing a permanently flexible fill. Will not crack, crumble or fall out
as the joint expands and contracts over time.  Superior paintability – won’t flash paint. Available in smooth grade and textured grade to provide the perfect finish.  Interior and exterior use.
Suggested Uses: Ideal for filling and repairing recurring cracks and holes in:
• Stucco
• Brick
Surface Preparation &amp; Application:
• Clean surface removing all loose particles, plaster, wax, dust, dirt, peeling paint, old caulk, grease and oil.
• Using a caulking gun, brush, putty knife or trowel, fill crack with patching compound.  Tool patched surface to desired finish before product dries.

Product is not sandable.

• For some repairs, more than one application may be required. Use product to bridge gaps up to 1/4” wide and 1/2” deep.  Fill small cracks and surface defects to a minimum depth of 1/8”.
• Can be painted after 30 minutes with latex-based paints. Allow to dry at least 2 hours (longer in cool or humid conditions) before painting with oil-based paints.
• Wash tools immediately after use with water.