When it comes down to it, there are 2 ways to make money (ec) in STO:
1) Spend real money (buy Zen with cash, then buy keys/doff packs/fleet ship modules and sell them on the exchange)
2) Grind
The reason most people aren't rich on STO is because they would rather play than sell stuff (I actually enjoy messing with the market, which is why I'm generally loaded, but I know that's not for everyone ;) )
That said, there are some things you can do that don't take an absurd amount of time that can turn a healthy profit (or at least healthy enough for you to be able to get some pretty things every once in awhile)
The suggestion about colonization Doffs is a good one, in general, breaking down purple Doffs in to their components can net you a solid amount of cash, though using the colonization Doffs can be time consuming if you don't have Doffs that are good at criting those missions. So the lesson here is, any purple Doff will do (and breaking down blue ones isn't a losing proposition either). So here are some ways to get some blue/purple Doffs pretty easily:
1) Caitain Field Flight Training Doff Chain: This one will show up once a day after you've finished the chain and will award a Doff upon completion of the repeatable mission. The result is usually blue, and sometimes purple. A blue Doff breaks down into 9 white Doffs, which is roughly 250,000 ec when you sell them on the exchange.
2) Fleet Starbase: The personnel officer will sell you a random purple Doff for 25,000 fleet credits (it's a bit steep, but if you're rolling in fleet credits and want to change them to ec, this is an option). Do make sure to check the Doff itself on the exchange before you break it down, sometimes you'll get really good ones worth between 5-10 million ec, so breaking it down to sell whites to get 800-900k is kind of a losing proposition.
3) Buying them off the exchange. I know, this seems counter intuitive, but if you buy a crappy Doff, like a Warp Theorist or a Flight Deck Officer for 300-400k and then break it down and sell the pieces for 800-900k, that's generally worth the effort.
Something to note while breaking down Doffs... pay attention to what is valuable. For me, there's not many that are worth worrying about, because the margin of error is low (i.e. a green trader is worth 100k, but the 3 whites will be worth between 60-100k, so it's not worth worrying about). But there are a few that are definitely worth paying attention to: Blue (and green) technicians, the blues generally go for 1-3 million a piece; Blue damage control engineers, around 750k; Blue Research Lab Scientists, 500k; Blue Fabrication Engineers, 350k.
Another unfortunate reality of STO's economy is that it works like any other capitalist economy, the more money you have, the easier it is to make more. There are some ventures that are high risk/high reward in small quantities, namely opening doff and weapon packs, and to a smaller extent lockboxes, that flatten out in larger quantities. If I spend 5 million ec on Doff packs and open them up and sell the contents, I will probably lose some money, but if I spend 100 million ec and do the same thing, I will probably make 10-15 million more than my original investment off the deal (and upwards of 200 if I get lucky).
It's all about prioritizing what you want to spend your time doing, and for most people, that's playing the game, which is a great choice. In those cases, selling drops helps, and playing the elites so you can sell salvaged tech is a sustainable way to make sure you have enough money to function. If you want to buy the big stuff, and don't want to spend real cash, try setting aside a little bit of time each time you play to work on getting money. Breaking down a purple Doff and listing it all on the exchange will only take like 10 minutes, so if you do that each couple of days, you'll end up ok.
A couple other random tips:
1) Make sure you clear out your exchange notifications from your mail. I used to sell so much stuff that when the mail server broke on the launch of New Romulus, it completely crashed by game because I couldn't go delete my messages (I finally created an account on another PWE game just so I could log in and clear my mail).
2) Don't spend too much time pricing out the white doffs, it will make your life miserable if you try to look up each doff individually. Establish a baseline and sell all doffs of that type for the same price (i.e. look up a random tactical doff, and then use that as your price for all tac doffs, be it energy weapons officer or projectile weapons officer).
Happy money making!