The most important skill developed during my time as FO so far has been to become a Cameleon. That is to say, ensure that I find something in common with the other driver and do my best to create a positive flight deck atmosphere. Of course there are people that can make this can be bloody hard work, but thus far I've found no skipper has failed to listen to a suggestion or hint if I see a balls up developing. Like wise, if I've cocked up I don't become all defensive. Just reply "s**t, thanks mate", and swiftly sort it out.
I understand that the captains authority is final, however creating the right atmos from the outset makes life alot easier if/when you need to do something.
I know this all sounds a bit soft and "group hug-ish", but if I see a guy about to plough me into the ground I need to be able to do something about it, and a good rapport with your colleague goes along way!