Dot notation is RECOMMENDED over bracket notation for getting and setting properties.

For Example:

view.backgroundColor = [UIColor orangeColor];
[UIApplication sharedApplication].delegate;

Not:

[view setBackgroundColor:[UIColor orangeColor]];
UIApplication.sharedApplication.delegate;

• Indentation MUST use 4 spaces. Never indent with tabs. Be sure to set this preference in Xcode.
• Method braces and other braces (if/else/switch/while etc.) MUST open on the same line as the statement. Braces MUST close on a new line. For Example:

if (user.isHappy) {
    // Do something
}
else {
    // Do something else
}

• There SHOULD be exactly one blank line between methods to aid in visual clarity and organization.
• Whitespace within methods MAY separate functionality, though this inclination often indicates an opportunity to split the method into several, smaller methods. In methods with long or verbose names, a single line of whitespace MAY be used to provide visual separation before the method’s body.
• @synthesize and @dynamic MUST each be declared on new lines in the implementation.

For example:

if (!error) {
    return success;
}

Not:

if (!error)
    return success;

or

if (!error) return success;

When methods return an error parameter by reference, code MUST switch on the returned value and MUST NOT switch on the error variable.

For example:

NSError *error;
if (![self trySomethingWithError:&error]) {
    // Handle Error
}

Not:

NSError *error;
[self trySomethingWithError:&error];
if (error) {
    // Handle Error
}

Some of Apple’s APIs write garbage values to the error parameter (if non-NULL) in successful cases, so switching on the error can cause false negatives (and subsequently crash).

In method signatures, there SHOULD be a space after the scope (- or + symbol). There SHOULD be a space between the method segments.

For example:

- (void)setExampleText:(NSString *)text image:(UIImage *)image;

When accessing the x, y, width, or height of a CGRect, code MUST use the CGGeometry functions instead of direct struct member access. From Apple's CGGeometry reference:

All functions described in this reference that take CGRect data structures as inputs implicitly standardize those rectangles before calculating their results. For this reason, your applications should avoid directly reading and writing >the data stored in the CGRect data structure. Instead, use the functions described here to manipulate rectangles and to >retrieve their characteristics.

For example:

CGRect frame = self.view.frame;

CGFloat x = CGRectGetMinX(frame);
CGFloat y = CGRectGetMinY(frame);
CGFloat width = CGRectGetWidth(frame);
CGFloat height = CGRectGetHeight(frame);

Not:

CGRect frame = self.view.frame;

CGFloat x = frame.origin.x;
CGFloat y = frame.origin.y;
CGFloat width = frame.size.width;
CGFloat height = frame.size.height;

Values MUST NOT be compared directly to YES, because YES is defined as 1, and a BOOL in Objective-C is a CHAR type that is 8 bits long (so a value of 11111110 will return NO if compared to YES).

For an object pointer:

if (!someObject) {
}

if (someObject == nil) {
}

For a BOOL value:

if (isAwesome)
if (!someNumber.boolValue)
if (someNumber.boolValue == NO)

Not:

if (isAwesome == YES) // Never do this.