I completely agree with x. For one thing this game is based around flow resources so if you're buying a superfortress for 1.6 million metal or whatever (don't remember cost exactly) and you can't even have that many resources stockpiled at once, it doesn't matter because of how the flow is working. If you find your 'sweet-spot' difficulty level, which can be difficult in itself because your skill is always changing, you should theoretically never have resources. There is a wealth of things to build, depending on different factors such as if you're buying from traders, using mercenaries, or what you have researched. I find I'm always struggling for resources, which to me is a good thing. (I too hate aspects of games such as C&C where it's a bit of a struggle at the beginning until you hold off the enemy for just long enough that you have a massive force to wipe them out.)
Like Chris said, if you are constantly reaching your resource cap you're doing something wrong. Try building an Ion Cannon somewhere where it will be effective against the AI, still have credits? Buy a MkII Ion Cannon. Max out your fleet, use mercenaries to get that bit of extra boost to your fleet you might be needing. Think that's a waste of credits? Then research MkII-III shield generators and have the strongest ones around your main space stations and use weaker ones to cover important buildings/targets and use some to block wormhole entrances to your capital planet. Build a superfortress, build shield generators around your mining facilities, continually send out your main fleet to assault AI locations then use credits to replenish your forces, research more turrets and load your planets with turrets, tachyon's, tractor beams, mines. I could go on forever, I honestly can't see how anyone could reach their cap unless they went for lunch and came back, or something. Worst case for me is if I come back and have 2-3k in resources after focusing on something else such as a large battle or invasion, etc, which I find I've spent in the next few minutes.
I even find myself needing to mildly micromanage my power generators to squeeze out as much resources as possible from them. (I used this kinda reversed as generators don't produce resources, save energy.) Example: You start with a MkII generator which produces 40,000 energy. If you are building a fleet, buildings, etc, you'll soon need to build a MkI=5,000, MkIII=80,000, which gives a total gross of 125,000 at the cost of a total of -60 m+c. This is fine and dandy at the start but when you take over say two more planets you shouldn't necessarily jump ahead and build 2 more MkIII generators, which is -80 m+c between both of them and 160,000 energy. My rule of thumb is to stick with as many of the smallest generators neccessary, depending on your energy needs, and to turn off any extra you don't need. Without shutting anything down you're producing 285,000 gross energy. Say you've got a couple energy-comsuming 'things' and have 90,000 net energy. That's quite a waste in energy. If you were to build two more MkII generators, one at each of your two newly aquired planets together they would produce 80,000 energy and cost a total of -30 m+c. You then shut off one of your MkIII generators giving you back 40 m+c. You are now making an extra 10 m+c for running on lower-generators. Of course once you have more limited planets you may have to resort to using all generators. Another alternative is to use some extra MkI genrators here and there, even if they're ineffecient, as long as they're not overused. ie: you still have 90,000 extra net energy so if you shut down your MkIII energy generator at your home planet, saving you 40 m+c then build another MkI generator giving you 4,000 energy but costing you only -2 m+c, even though you only have 14,000 energy you're now making a fair bit of extra materials to assist in your building. I generally try to keep my energy between 10-20k unless I'm planning on building something big, which reminds me you can also power down your starship factories and other similar buildings you're not currently using, then in turn power down a generator or two boosting your income, so long as you remember how to reverse it when you need to use those buildings again.
Hopefully that isn't too elementary or too much of a review for you, for some reason I just had to get out one of the strategies I like to use.
EDIT: Just a quick sum-up of the energy thing:
MkI x16 -32 crystal, -32 metal 80,000 energy
MkII x2 -30 crystal, -30 metal 80,000 energy Actually looks like too many MkI's aren't quite as efficient as MkII's in the long run
MkIII x1 -40 crystal, -40 metal 80,000 energy
