Here's an idea that might have merit: Allow a gem store subscription that grants discounted prices all the time, rather than only when "someone" decides to have a sale.
Make the discount smaller than the usual discount (10% vs. 20% or more), so regular sales stay an option. But, allow it on more items in the list. The consistency of pricing would probably increase overall sales AND ANet would already be making something from the subscription itself.
It might need to be capped, though. For example, $10 per month might buy you 10% off 16,000 gems worth of stuff. This example would be about the same as typical 20% discounts (if the player spent it all), but they could be had whenever the player wanted.