Many players are more adept at earning Energy Credits than they are anything else. If anything, once you become a Vice Admiral, the need for Energy Credits goes down. You can get most anything you want through enough play and a bit of luck. Moreover, as you go through the end-game content, you will continue to earn drops and other rewards.
You will need to make use of the Exchange in order to really earn credits, however. Keeping track of changes in the game will also help you find needs other players may have. This is important, as the more necessary the item is, and the more inconvenient it is to get, the more its potentially worth. Additionally, if things fluctuate in value, keeping track of when it might be cheapest to buy is time well spent.
One of the most relevant pieces of news, for instance, is when Cryptic has Master Keys on sale! Right now until Thursday, December 27th, Master Keys are 15% off, and the prices of Master Keys on the Exchange have gone down accordingly. Instead of paying 1.4-1.5 million EC per key, its possible to get them for 1-1.1 million EC instead.
For further reading on ideas to make more Energy Credits: