Debugger

Edit the breakpoint, add debugger commands:
p // prints out contents of a primitive
po // prints out contents of an object
po (NSException *)$eax
expr // expression
expr NSLog("somecomment: ", object)
c // continue after breakpoint

Build Configuration

Toggle between Debug and Release builds:
Choose Product > Scheme > Edit Scheme. Change the Build Configuration under the Info tab.
Shortcut: hold Alt ⌥ and click the run button ▶.

Zombies

Environment variables such as Zombies are now part of the "scheme".
Product > Edit Scheme

"Run Foo.app" stage in the left panel, and the "Arguments" tab on the right. You can then add NSZombieEnabled to the "Environment Variables" section and set the value to YES.

In Xcode 4.1 and above, there's also checkbox on the "Diagnostics" tab of the "Run" stage to "Enable Zombie Objects".

To debug and find the source: Enable Zombies for the app (Product\Scheme) and Launch Instruments, Select Zombies. Run your app in Xcode Then go to Instruments start recording. Go back to your App and try generating the error. Instruments should detect bad call (to zombie) if there is one.

Carthage

Dependency manager for Cocoa.
carthage update // Fetch dependencies into a Carthage/Checkouts folder and build each one.

xcodebuild

Building from the command line

xcodebuild -list -project [project].xcodeproj // list project details

Build project via Scheme:
xcodebuild -scheme [scheme] build

iOS 8 runtime headers in GitHub for debugging Xcode