I've played this since Marchish, and have been with the Black Knights for awhile. Anywho, a good way to grow is to raid the countless inactive nations with no military with tons of stocked up money and resources. Just be sure not to raid any alliances at all, just don't do it you'll get countered, receive bad rep and likely pay the alliance back in reparations. As well, there is a fair bit of money to be made in brokering on the Orbis market, I used to do this but stopped due to time constraints but it can be lucrative. A little crappy guide on brokering, since I'm by far not the best but it's the general gist.
So, here it is.
You can set up 2 forms of offers, an offer to buy a resource and an offer to sell a resource.
You basically, set up an offer to buy a resource whose highest "bid" has the greatest difference with the lowest offer to sell. This is your profit for every unit you buy and sell, and it stacks up.
The offers to buy are generally lower than the to sell offers because people sell you (the buyer) at a lower price for instant cash on hand for their side. You can probably tell, this can make you a pretty penny fairly quickly depending on market activity.
The best resources to broker are the war resources, munitions, steel, aluminum and gas especially during war time, which is right now. This is mostly due to the activity of these resource markets, allowing you to gain a profit faster and reinvest it into brokering.
An that's it, you can probably ask someone in your alliance about brokering who will be loads more experienced than myself, but this starts you off while you stay unaligned.