@property (copy, nonatomic) NSString *someString;

- (void)setSomeString:(NSString *)someString
{
  //option 1
  _someString = someString;
  //option 2
  _someString = [someString copy];
  //do other stuff
}

@property (copy, nonatomic) NSString *someString;

_someString = [someString copy];

if(_someString) {
    [_someString release];
}