I want to suggest Double XP weekends for paying and non paying customers. I know from prior games, like Diablo and Call of Duty, it seemed to bring in more people for longer time frames.
Also for the players that have no time to grind out the levels you should make an option to purchase Exp Baubles. Do it thru the shop. Still making money regardless. Just some thoughts from a player from 1997. Dont mind me. :-)