gistfile1.m

@interface MyClass : NSObject {
    iVar *aProtectedIVar;
    @public
    iVar *aPublicIVar;
    iVar *aSecondPublicIVar;
    @protected
    iVar *aSecondProtectedIVar;
    @private
    iVar *aPrivateIVAr;
}

@end

gistfile2.m

/*We declare the category*/
@interface  MyClass (private)

@property (nonatomic, retain) Type *aPrivateProperty; 
- (void)aPrivateMethod;

@end

/*We implement it*/    
@implementation MyClass (private)

/*We cannot use synthesize in the category :-(*/
@dynamic aPrivateProperty;
- (Type *)aPrivateProperty {
    //Getter code
}

- (void)setAPrivateProperty {
    //Setter code
}
    
- (void)aPrivateMethod {        
    //Some code there
}
@end

gistfile3.m

/*We declare the class extension*/
@interface  MyClass () 

@property (nonatomic, retain) Type *aPrivateProperty; 
- (void)aPrivateMethod;

@end

/*
We can use the main implementation block to implement our properties
and methods declared in the class extension.
*/
@implementation MyClass

/*Therefore we can use synthesize ;-)*/
@synthesize aPrivateProperty;

- (void)aPrivateMethod {        
    //Some code there
}

@end

gistfile4.m

@interface MyClass : NSObject {
    //iVars declaration
}

/*We declare the property as readonly in the header*/
@property (readonly, nonatomic, retain) Type *aProperty;
 
@end

gistfile5.m

@interface MyClass () 

/*
We redeclare the property in the .m file but this time with readwrite access.
When accessed from inside an instance of this class you can set the value of the
property, when accessed from outside you can only read its value.
*/
@property (readwrite, nonatomic, retain) Type *aProperty;
@end

gistfile6.m

@interface TheMechanics : NSObject {
    @private
    NSArray *_tools;
}

- (void)repairCar:(Car *)aCar;

@end

gistfile7.m

#import "TheMechanics.h"

@interface TheMechanics ()

- (void)useToolOnCar:(Car *)aCar;
@property (nonatomic, retain) NSArray *tools;

@end

@implementation TheMechanics

@synthesize tools = _tools;

- (void)repairCar:(Car *)aCar {
    
    for (Tool *tool in self.tools) {
        [self useToolOnCar:aCar]
    }
}

@end

gistfile8.m

TheMechanics *mechanics = [[TheMechanics alloc] init];
[mechanics repairCar:self.car];
[self lookingBack:OverMyShoulder];