So you cant focus on one character...because you have leveled a few to 50 or just bcause you get easily bored?
Just get an XP-boost as said above.
Me, Im just so immersed in my character...yesterday I looked at him (yes Im new in the game) and he was 19 when I thought he was only 15.
leveling is so easy I think...but Im more of a 1-2 characters-guy...no power-leveler that will have 6 chars on 50, ever...