Bi-monthly promos are excessive because it takes the meaning out of "exclusive items". Why hold raffles instead?
For every $X you spend on the game, you get a raffle ticket. You can go to a special NPC to submit it or save up for a future raffle.
At the end of every month a random ticket is chosen and the winner is given a valuable accessory and a lump of energy.

This provides incentive to spend more money while throttling the crown sinks going into the system. It can also be a fun forum event that happens every month when the winner is announced. Raffle tickets come as a gift along with purchases so the "losers" don't really lose out since they still got what they paid for.