Just a common sense tip-
When you have the flag, take it either ramp/tunnel and go out towards your graveyard trying your best to stay out of sight. Have someone there waiting there for you at your graveyard. By this time you most likely have a few people from the other team on you. Go into defensive stance and intervene your friend (bloodrage if needed), taking you straight up your graveyard and out of harm. This works great and no one expects it. I scored all 3 flags 5 games in a row with my premade and I found this to be one of the most effective ways.
-Ulgrim