Ok, well first off, you need a flying mount. What you do is you get on your flying mount and be right outside the instance portal (Note this only works with instances that doors are either on the outside or where u can be mounted). What you do is you fly straight up in the archway that the portal is in (all instance portals have an archway or some ceiling). If you are flying straight up, it will appear as though part of you is going through the ceiling. Then you gently tilt forward, so you are still basically going straight up, but you will start to go forward. This will get you PAST the areatrigger (what forces you to zone), and then you are past the portal. Dismount or continue on ur mount, and vouala, your past it.
NOTE: Many instances still have an invisible wall past the portal, so you cannot go far, also this has only been tested for a few instances, as testing it for all would be rediculous