I disagree with the advice to stay off other people's routes. Just saying that a route belongs to someone is silly. In my game play, it's a free for all and competition makes it fun. No need to tip-toe around other players.
At the same time, I'll also accept the consequences if the competitor turns around and shows me who's who for doing so. I'm just careful who I think I can compete against and if it's worth it.
I would have to agree with the above statement. Mostly because any city worth flying to, is going to have competition. Any city that no one flies to, will in most cases only result in a loss of revenue. So why are so many "veterans" suggesting to new people that they should lose money? Unless you want new players to be using $10 planes that carry less than 50 people at a time, we newbies have to join the fight too.