With iOS 5+ it’s as easy as:
UIBarButtonItem *btnShare = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemAction target:self action:@selector(share)];
UIBarButtonItem *btnRefresh = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemRefresh target:self action:@selector(refresh)];
[self.navigationItem setRightBarButtonItems:[NSArray arrayWithObjects:btnShare, btnRefresh, nil]];