Bring another round of the Rose Regalia Bonus Sale. I'm betting that you've already planned on doing so, but here's my addition so it'll make a bigger splash.
Part 1
Don't recycle the same art from the previous Rose Regalia Set. This way both sets will retain their aesthetic values. Also, this will ensure that the players know future sets will not be the same thus not decreasing interest. Options for design: 1. revamped art on the current item 2. or entirely new theme and/or 3. same or new set of three colors.
Part 2
Time the sale for when the Crystal Energy via Crowns returns to a steep climb like the recent Saturday to Tuesday prices. Since the price has plateaued for the time being the sale should wait for the most effective time. If the sale launches during a price spike most players would be inclined to purchase Crystal Energy via Cash. On top of wanting to already buy Crystal the sale would greatly 'persuade' players to purchase higher quantities than they previously would have had if there was no sale.
Part 2.5
The other reason for timing it during a price spike is to stabilize the Crystal Energy market making it less painful (so they don't decide to quit) for the players who fall into the category of half Crystal via Cash and half Crystal via Crowns and those who fully rely on Crystal via Crowns.
The rose set was terrible for stabilizing the CE market
Straight after the promo was over, CE prices skyrocketed to prices higher then prior to the promo almost immediately.
i have no idea why that is so, but it was amusing to me at least =p