First of all, pick some economy bonuses when choosing your race, abilities and political party.
Secondly, it helps to spend some ability points on either a morale boost or extra population growth.
Then during the game itself:
- Don't grab planets of planet quality 6 or lower, especially not if you already have better planets in the same star system. Those small planets will come under your cultural influence later on and flip to your side.
- Don't overdo the expansion because indeed it hurts economically. You can as well let the AI develop some nice planets so that you can take them over fully grown

- Cash anomalies which your survey vessel comes across (especially the 1000 bc ones help a lot)
- Clever research order: some extra research, population growth, economy bonuses early on will soften the economic blow
- Choose what you're building and in which order: all factories is a viable strategy but you need to be prepared for it. Otherwise you're better off adding also some economical & morale buildings on your homeworld. Get to the Econ Capital quite early.
- Tech trading: sell xeno communications, universal translator and some other quite basic techs to all AI's, especially minors, for good cash.
- Construct starbases on resources to get bonuses
- Trade routes don't help that much (especially in Twilight of the Arnor) but every bit helps.
- Before you advance to other government types, happiness is only important as an influence on each planet's birth rate. Early on, aim at 100% happiness to grow population faster. Then with for example 2000 bc left, move to 76% (another breaking point in the relation between happiness and population growth rate), then even to around 50%.
Take into account that at 46% happiness or below (if I remember the exact figure) the population of that planet stops growing.
Once you move on to other governments (to get the according bonuses), make sure to have at least 60-65% happiness just before each election. Inbetween elections, only the effect on population growth matters.
Cutting social spending will stop your planets from building any further or upgrading any buildings. Typically this is only good if:
- you need fast focus on research and/or military production
- you don't have any buildings or upgrades in queue anymore
Otherwise it puts you in a bit of a standstill. I'd rather cut military expenses to improve your income or speeden up your building.