At around level 52 you will finish up in Lavastorm and you can move on to the Desert of Flames zones. In particular Sinking Sands, though heading to Pillars of Flame and doing some quests can reward you with some good XP. Stay in Sinking sands until around 57. Start by doing the quests where you zone in with the carpet then work your way around the zone. If you reach a level 57 before you completed all the quests you should turn the AA slider to 100% before completing the rest of the quests
The map below shows the different quest givers and in approximately what order they should be done: