One thing I always try and do is, once the colony rush phase is wrapping up, set taxes as low as possible so as to have 100% happy on every planet I’ve got (or at least the large ones) to have a population boom. Then turn the taxes back to “an inch below global revolt” once the planets are near full. Also I try to nab every green space resource I can because that’s even moar money. That provides a good tax base for the rest of the game. Works even better if you’re the space frogs or a custom Super Breeder race obviously.
Also, do turn off auto-upgrading buildings on your planets and micro manage that shit yourself. As **Hoopy **says, higher tier buildings come with higher tier upkeep and upgrading too much too fast can put you deep in the red. Also, unless your planets have +happy squares or you want to use the planet as a culture bomb, don’t build farms at all on them. Ever. Yes, on paper more pop = more taxes, but it also means more unhappy to deal with with costly entertainment buildings, and since taxes are set on a national level one unhappy planet can force you to drop taxes empire-wide. In my experience, you always make more money with one more bank than with more people.
Try to keep tech trading to a minimum, and when you do try and trade that tech to everyone in the same turn. The reason is, AI civs tech trade like crazy and if you sell a tech no one has, they will pawn it to your mortal enemy as soon as your back is turned. So you might as well do it yourself and get some moolah out of it.
For warfare, try to build ships tailored to mess up those of your current enemy(ies), but also try to keep your armor ratings as multiple of 4. The reason is that armour of one type shot at by another type of weapon only counts as 1/4th of its value rounded down, so to a missile, 4 points of ballistic armour and 6 points are exactly the same.
Miniaturization means you can pack more crap onto each chassis, but also that each fighter produced is more expensive, slower to build and you lose more when it gets blown up. I find that Logistics is a better way to get more bang, by means of having more inferior fighters fighting as one unit. If you’re the Arceans (or more likely a custom civ with Super Warrior, because Arceans suck ass), it’s even more important because Super Warrior means you want to make large fleets of small ships that pack nothing but weapons.
Ground assault cannot take place if there is even one fighting ship orbiting the planet. In a pinch, rush buying a Tiny ship with a zip gun can save an entire planet from Korath death bombing.
The stock constructors and miners are over-designed. Too much in the way of engines and life support. As soon as you’ve got Small chassis, design some Small boats with just the construction module and nothing else, or one life support if you’ve nabbed resources far out in the black. Faster to build, cheaper to upkeep. Takes a longass time to get anywhere, but most of the time you’re using constructors to either build stations within the system, or upgrade existing stations so speed is not essential.
I also like to build Freighter chassises with one engine and then as many sensors I can cram onto them. Then park those on the edges of my empire, AWACS style. Ain’t nobody sneaking a fleet past.
As of the second expansion, spies are a complete waste of money. Either turn them off entirely, or only build 4 to be able to get maximum data on each civ consecutively and forget about them entirely until someone is sending spies at you. The reason is, each spy you make costs more than the last so if you send 10 at somebody, he can buy 10 to get rid of them for the same price, then the next civ can send 10 at you and suddenly your own 10 counterspies cost a mint.