When to retain an NSString?
-forExample:(NSString*)str{
    [str retain];

    //do something

    [str release];
}

- (void) forExample: (NSString *)theString {
    // Will need this object later. Stuff it into an ivar and retain it.
    // myString = [theString retain];
    // For NSString, it's actually better to copy, because this
    // could be a _mutable_ string, and it would in fact be best to use
    // the setter for the ivar, which should deal with the memory management:
    [self setMyString:theString];

    // Do things...

    // Don't release.
}

- (void) otherExample {
    [myString doYourStringThing];
    // If you don't need the string past this point (which would be slightly
    // unusual -- it leaves an ivar unset) release it and set it to nil.
    // [myString release]; myString = nil;
    // Again, it would be best to use the setter:
    [self setMyString:nil];
}

// Generally you keep ivars around until the instance is deallocated,
// and release them in dealloc
- (void) dealloc {
    [myString release];
    [super dealloc];
}

- (void)myFunction:(NSString *)foo block:(void (^)())callback {
    [foo retain];
    callback();
    // .. do some stuff
    [foo release];
}

- (void)myCallingFunction {
    NSString * myVariable = [[NSString alloc] initWithString:@"Test"];
    [self myFunction:myVariable block:^ {
        [myVariable release];
    }];
}