The point is to have an economy strong enough to support buying the alliance of a few City-states. This means earning at least 50-75 gold per turn so you can give them the 500 gold they need to become allied with you each time it is required. The figure I mentioned is in my opinion the bare minimum to sustain 3 CS : that's what I've done in my last game and that was just enough to still have a bit of pocket money to buy a few hex, a reinforcement unit or something like this. Then, you should discover all the advantages there is to be good friend with a CS: they give you food, culture or military units and all their interesting luxury resources, which should pretty much alleviate your happiness problems. At least in my last game, I didn't have to build any happiness building for a very long time due to this (and a bit of luck in the map which allowed me to have a lot of different happiness resources by myself or from trade). Note that I haven't tried the militaristic way of playing that much (well I've done it once I beelined to mechanized infantry...), so this might not quite apply to your case. There is no secret to have a decent economy: spamming trading posts is the way to go, with money buildings all along the way. The silver, gold and gems resources are quite useful too. Then, you should focus your cities toward some particular specialities (science, hammer, money, great persons...) and only build the appropriate buildings so you won't spoil any money to sustain the useless ones.