Find out the quest reset times and do the event quests daily.
Remember to activate the Double XP, usually a momento.
Use training gear and XP levelling potions to maximize your XP gains
Make use of the Double XP to level your low level characters by running Normal random dungeons and doing daily writs
Do the event related special quests daily. They grant XP and most importantly, event-related goodies such as rare recipes, costumes and more!
Remember, XP does not grant skill points after you have reached level 50 so you’ll need to collect sky shards, do skill-point related quests and complete new dungeons!