In Galaxy Empire for the iPhone, the three resources that you have to collect are metal, gas and crystals. All three of them are necessary to literally everything that you do in the game. You can get them from mines, but you also need energy to get them from mines, or else the mines will not run at full speed. So how do you get more metal, gas, crystals and energy in spades? Read on!
Aside from leveling up your mines as high as you can take them, there are a few ways to load up. One way is to get into a huge ship battle with someone else. Whether you attack them or they attack you, if you catch it soon enough, send your recyclers over to their planet (or to your own planet if you got attacked) and start recycling all of the debris. The more recyclers, the better, because one recycler can never get it all.
Send multiple large cargo ships with you not only when you fight, but especially send them to the mysterious nebula as often as you can, which is usually a specified amount of times per day. If you use up the amount of nebula visits before it becomes a black hole, then start visiting mysterious nebulae in other galaxies to get it from them, too.
To get more energy, aside from upgrading the solar plant (AKA: the obvious), built a fusion reactor as soon as you get the chance. To do this your gas mine needs to be upgraded to at least level 5, and your energy tech needs to be at least level 3.
Research your energy tech over and over and over again, as high as you can take it, because the more research you put into energy tech, the less energy you need to run your place. Also, go to the shipyard and construct a bunch of Solar Satellites. Make sure that they are well defended, though, as they are pretty much hopeless against any sort of attack.