The key factor these days is your Status in their Frequent Flyer scheme. The days of easy upgrades are long gone.
Not necessarily. A certain European carrier will definitely NOT upgrade frequent flyers every time. This is to prevent the wise ones from booking economy on flights that are busy or oversold. Instead, fancy algorithms calculate who is the most valuable to the airline. Rather predictably it means people who are not sure what to do with a salad and can only eat with their hands get to the posher area up front.
PM